Use a shared example, and a describe block per field

This also fixes todo-ed rubocop violations in graphql_matchers

spec/graphql/types/snippets/blob_type_spec.rb

@@ -14,26 +14,37 @@ RSpec.describe GitlabSchema.types['SnippetBlob'] do
     expect(described_class).to have_graphql_fields(*expected_fields)
   end
 
-  specify { expect(described_class.fields['richData'].type).not_to be_non_null }
-  specify { expect(described_class.fields['plainData'].type).not_to be_non_null }
-  specify { expect(described_class.fields['rawPath'].type).to be_non_null }
-  specify { expect(described_class.fields['size'].type).to be_non_null }
-  specify { expect(described_class.fields['binary'].type).to be_non_null }
-  specify { expect(described_class.fields['name'].type).not_to be_non_null }
-  specify { expect(described_class.fields['path'].type).not_to be_non_null }
-  specify { expect(described_class.fields['simpleViewer'].type).to be_non_null }
-  specify { expect(described_class.fields['richViewer'].type).not_to be_non_null }
-  specify { expect(described_class.fields['mode'].type).not_to be_non_null }
-  specify { expect(described_class.fields['externalStorage'].type).not_to be_non_null }
-  specify { expect(described_class.fields['renderedAsText'].type).to be_non_null }
+  let_it_be(:nullity) do
+    {
+      'richData' => be_nullable,
+      'plainData' => be_nullable,
+      'rawPath' => be_non_null,
+      'size' => be_non_null,
+      'binary' => be_non_null,
+      'name' => be_nullable,
+      'path' => be_nullable,
+      'simpleViewer' => be_non_null,
+      'richViewer' => be_nullable,
+      'mode' => be_nullable,
+      'externalStorage' => be_nullable,
+      'renderedAsText' => be_non_null
+    }
+  end
 
   let_it_be(:blob) { create(:snippet, :public, :repository).blobs.first }
 
-  described_class.fields.each_value do |field|
-    it "resolves #{field.graphql_name} using the presenter", :request_store do
+  shared_examples 'a field from the snippet blob presenter' do |field|
+    it "resolves using the presenter", :request_store do
       presented = SnippetBlobPresenter.new(blob)
 
       expect(resolve_field(field, blob)).to eq(presented.try(field.method_sym))
     end
   end
+
+  described_class.fields.each_value do |field|
+    describe field.graphql_name do
+      it_behaves_like 'a field from the snippet blob presenter', field
+      specify { expect(field.type).to match(nullity.fetch(field.graphql_name)) }
+    end
+  end
 end

spec/support/matchers/graphql_matchers.rb

 # frozen_string_literal: true
 
+RSpec::Matchers.define_negated_matcher :be_nullable, :be_non_null
+
 RSpec::Matchers.define :require_graphql_authorizations do |*expected|
   match do |klass|
     permissions = if klass.respond_to?(:required_permissions)
@@ -90,7 +92,7 @@ RSpec::Matchers.define :have_graphql_arguments do |*expected|
     @names ||= Array.wrap(expected).map { |name| GraphqlHelpers.fieldnamerize(name) }
 
     if field.type.try(:ancestors)&.include?(GraphQL::Types::Relay::BaseConnection)
-      @names | %w(after before first last)
+      @names | %w[after before first last]
     else
       @names
     end
@@ -103,9 +105,10 @@ RSpec::Matchers.define :have_graphql_arguments do |*expected|
   end
 
   failure_message do |field|
-    names = expected_names(field)
+    names = expected_names(field).inspect
+    args = field.arguments.keys.inspect
 
-    "expected that #{field.name} would have the following fields: #{names.inspect}, but it has #{field.arguments.keys.inspect}."
+    "expected that #{field.name} would have the following arguments: #{names}, but it has #{args}."
   end
 end