Address specs failing in rails 4

We want to run CI with rails 4 for a short-term (until we are sure that
we will ship with rails 5). The problem is that rails 4 can not handle
rails 5 schema.rb properly - specifically `t.index` directive can not
handle multiple indexes on the same column.

Because combination of rails 4 + rails 5 schema will be used
only in CI for a short-term, we can just ignore these incompatibility
failures. This patch adds `rails5` helper for specs.

spec/lib/gitlab/database/migration_helpers_spec.rb

@@ -1338,7 +1338,12 @@ describe Gitlab::Database::MigrationHelpers do
   end
   describe '#index_exists_by_name?' do
-    it 'returns true if an index exists' do
+    # TODO: remove rails5-only after removing rails4 tests
+    # rails 4 can not handle multiple indexes on the same column set if
+    # index was added by 't.index' - t.index is used by default in schema.rb in
+    # rails 5. Let's run this test only in rails 5 env:
+    # see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21492#note_113602758
+    it 'returns true if an index exists', :rails5 do
       expect(model.index_exists_by_name?(:projects, 'index_projects_on_path'))
         .to be_truthy
     end

spec/lib/gitlab/github_import/importer/pull_request_importer_spec.rb

@@ -240,7 +240,12 @@ describe Gitlab::GithubImport::Importer::PullRequestImporter, :clean_gitlab_redi
           .and_return(user.id)
       end
-      it 'returns the existing merge request' do
+      # TODO: remove rails5-only after removing rails4 tests
+      # rails 4 can not handle multiple indexes on the same column set if
+      # index was added by 't.index' - t.index is used by default in schema.rb in
+      # rails 5. Let's run this test only in rails 5 env:
+      # see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21492#note_113602758
+      it 'returns the existing merge request', :rails5 do
         mr1, exists1 = importer.create_merge_request
         mr2, exists2 = importer.create_merge_request

spec/spec_helper.rb

@@ -235,6 +235,10 @@ RSpec.configure do |config|
     example.run if Gitlab::Database.mysql?
   end
+  config.around(:each, :rails5) do |example|
+    example.run if Gitlab.rails5?
+  end
+
   # This makes sure the `ApplicationController#can?` method is stubbed with the
   # original implementation for all view specs.
   config.before(:each, type: :view) do