Remove duplicate JSON schema matcher

We had three JSON schema matchers:

1. `match_schema` - takes a data structure and a path to a schema file.
2. `match_response_schema` - takes a response object and a path to a
   schema file.
3. `according_to_schema` - chained matcher on `be_valid_json`, takes a
   schema literal.

The reason for this is that we wanted to operate on various axes:

1. Are we validating a parsed data structure, or a potentially-JSON
   string?
2. Are we providing a path to a schema file, or a literal data structure
   representing a schema?

Happily, the library we're using already solves problem 1 for us.
https://github.com/ruby-json-schema/json-schema#validation says:

> All methods take two arguments, which can be either a JSON string, a
> file containing JSON, or a Ruby object representing JSON data.

So we just needed to solve concern 2, and we handle that by assuming
that if the schema argument is a string, it's not a JSON string
representing the schema, but a path to a schema file.

To handle the chaining that `according_to_schema` had, we can just use
RSpec's `and` method:

    be_valid_json.and match_schema(build_info_payload_schema)

doc/development/testing_guide/best_practices.md

@@ -842,6 +842,41 @@ Example:
 expect(response).to have_gitlab_http_status(:ok)
 ```
 
+#### `match_schema` and `match_response_schema`
+
+The `match_schema` matcher allows validating that the subject matches a
+[JSON schema](https://json-schema.org/). The item inside `expect` can be
+a JSON string or a JSON-compatible data structure.
+
+`match_response_schema` is a convenience matcher for using with a
+response object. from a [request
+spec](testing_levels.md#integration-tests).
+
+Examples:
+
+```ruby
+# Matches against spec/fixtures/api/schemas/prometheus/additional_metrics_query_result.json
+expect(data).to match_schema('prometheus/additional_metrics_query_result')
+
+# Matches against ee/spec/fixtures/api/schemas/board.json
+expect(data).to match_schema('board', dir: 'ee')
+
+# Matches against a schema made up of Ruby data structures
+expect(data).to match_schema(Atlassian::Schemata.build_info)
+```
+
+#### `be_valid_json`
+
+`be_valid_json` allows validating that a string parses as JSON and gives
+a non-empty result. To combine it with the schema matching above, use
+`and`:
+
+```ruby
+expect(json_string).to be_valid_json
+
+expect(json_string).to be_valid_json.and match_schema(schema)
+```
+
 ### Testing query performance
 
 Testing query performance allows us to:

spec/lib/atlassian/jira_connect/client_spec.rb

@@ -114,7 +114,7 @@ RSpec.describe Atlassian::JiraConnect::Client do
     end
 
     let(:body) do
-      matcher = be_valid_json.according_to_schema(schema)
+      matcher = be_valid_json.and match_schema(schema)
 
       ->(text) { matcher.matches?(text) }
     end
@@ -164,7 +164,7 @@ RSpec.describe Atlassian::JiraConnect::Client do
     end
 
     let(:body) do
-      matcher = be_valid_json.according_to_schema(build_info_payload_schema)
+      matcher = be_valid_json.and match_schema(build_info_payload_schema)
 
       ->(text) { matcher.matches?(text) }
     end

spec/lib/atlassian/jira_connect/serializers/build_entity_spec.rb

@@ -23,7 +23,7 @@ RSpec.describe Atlassian::JiraConnect::Serializers::BuildEntity do
       end
 
       it 'is invalid, since it has no issue keys' do
-        expect(subject.to_json).not_to be_valid_json.according_to_schema(Atlassian::Schemata.build_info)
+        expect(subject.to_json).not_to match_schema(Atlassian::Schemata.build_info)
       end
     end
   end
@@ -43,7 +43,7 @@ RSpec.describe Atlassian::JiraConnect::Serializers::BuildEntity do
 
         describe '#to_json' do
           it 'is valid according to the build info schema' do
-            expect(subject.to_json).to be_valid_json.according_to_schema(Atlassian::Schemata.build_info)
+            expect(subject.to_json).to be_valid_json.and match_schema(Atlassian::Schemata.build_info)
           end
         end
       end

spec/lib/atlassian/jira_connect/serializers/deployment_entity_spec.rb

@@ -23,7 +23,7 @@ RSpec.describe Atlassian::JiraConnect::Serializers::DeploymentEntity do
       end
 
       it 'is invalid, since it has no issue keys' do
-        expect(subject.to_json).not_to be_valid_json.according_to_schema(Atlassian::Schemata.deployment_info)
+        expect(subject.to_json).not_to match_schema(Atlassian::Schemata.deployment_info)
       end
     end
   end
@@ -86,7 +86,7 @@ RSpec.describe Atlassian::JiraConnect::Serializers::DeploymentEntity do
 
         describe '#to_json' do
           it 'is valid according to the deployment info schema' do
-            expect(subject.to_json).to be_valid_json.according_to_schema(Atlassian::Schemata.deployment_info)
+            expect(subject.to_json).to be_valid_json.and match_schema(Atlassian::Schemata.deployment_info)
           end
         end
       end

spec/support/matchers/be_valid_json.rb

 # frozen_string_literal: true
 
 RSpec::Matchers.define :be_valid_json do
-  def according_to_schema(schema)
-    @schema = schema
-    self
-  end
-
   match do |actual|
-    data = Gitlab::Json.parse(actual)
-
-    if @schema.present?
-      @validation_errors = JSON::Validator.fully_validate(@schema, data)
-      @validation_errors.empty?
-    else
-      data.present?
-    end
+    Gitlab::Json.parse(actual).present?
   rescue JSON::ParserError => e
     @error = e
     false
@@ -23,8 +11,6 @@ RSpec::Matchers.define :be_valid_json do
   def failure_message
     if @error
       "Parse failed with error: #{@error}"
-    elsif @validation_errors.present?
-      "Validation failed because #{@validation_errors.join(', and ')}"
     else
       "Parsing did not return any data"
     end

spec/support/matchers/schema_matcher.rb

@@ -2,6 +2,8 @@
 
 module SchemaPath
   def self.expand(schema, dir = nil)
+    return schema unless schema.is_a?(String)
+
     if Gitlab.ee? && dir.nil?
       ee_path = expand(schema, 'ee')