Improve Static Site Editor feature spec

Add coverage for reading config from file
Also add coverage for all existing generated config values

spec/features/static_site_editor_spec.rb

@@ -6,18 +6,64 @@ RSpec.describe 'Static Site Editor' do
   let_it_be(:user) { create(:user) }
   let_it_be(:project) { create(:project, :public, :repository) }
 
+  let(:sse_path) { project_show_sse_path(project, 'master/README.md') }
+
+  before_all do
+    project.add_developer(user)
+  end
+
   before do
-    project.add_maintainer(user)
     sign_in(user)
+  end
+
+  context "when no config file is present" do
+    before do
+      visit sse_path
+    end
 
-    visit project_show_sse_path(project, 'master/README.md')
+    it 'renders Static Site Editor page with all generated config values and default config file values' do
+      node = page.find('#static-site-editor')
+
+      # assert generated config values are present
+      expect(node['data-base-url']).to eq("/#{project.full_path}/-/sse/master%2FREADME.md")
+      expect(node['data-branch']).to eq('master')
+      expect(node['data-commit-id']).to match(/\A[0-9a-f]{40}\z/)
+      expect(node['data-is-supported-content']).to eq('true')
+      expect(node['data-merge-requests-illustration-path']).to match(%r{/assets/illustrations/merge_requests-.*\.svg})
+      expect(node['data-namespace']).to eq(project.namespace.full_path)
+      expect(node['data-project']).to eq(project.path)
+      expect(node['data-project-id']).to eq(project.id.to_s)
+
+      # assert default config file values are present
+      expect(node['data-static-site-generator']).to eq('middleman')
+    end
   end
 
-  it 'renders Static Site Editor page with generated and file attributes' do
-    # assert generated config value is present
-    expect(page).to have_css('#static-site-editor[data-branch="master"]')
+  context "when a config file is present" do
+    let(:config_file_yml) do
+      # NOTE: There isn't currently any support for a non-default config value, but this can be
+      #       manually tested by temporarily adding an additional supported valid value in
+      #       lib/gitlab/static_site_editor/config/file_config/entry/static_site_generator.rb.
+      #       As soon as there is a real non-default value supported by the config file,
+      #       this test can be updated to include it.
+      <<-EOS
+        static_site_generator: middleman
+      EOS
+    end
+
+    before do
+      allow_next_instance_of(Repository) do |repository|
+        allow(repository).to receive(:blob_data_at).and_return(config_file_yml)
+      end
+
+      visit sse_path
+    end
+
+    it 'renders Static Site Editor page values read from config file' do
+      node = page.find('#static-site-editor')
 
-    # assert file config value is present
-    expect(page).to have_css('#static-site-editor[data-static-site-generator="middleman"]')
+      # assert user-specified config file values are present
+      expect(node['data-static-site-generator']).to eq('middleman')
+    end
   end
 end