Use schema to test resource access token list API

This change introduces a schema fixture to verify the response for the
list responses for Group and Project Access Tokens.

It also makes sure that the response contains the proper pagination
headers.

spec/fixtures/api/schemas/public_api/v4/resource_access_tokens.json

+{
+  "type": "array",
+  "items": { "$ref": "resource_access_token.json" }
+}

spec/requests/api/resource_access_tokens_spec.rb

@@ -29,6 +29,8 @@ RSpec.describe API::ResourceAccessTokens do
           token_ids = json_response.map { |token| token['id'] }
 
           expect(response).to have_gitlab_http_status(:ok)
+          expect(response).to include_pagination_headers
+          expect(response).to match_response_schema('public_api/v4/resource_access_tokens')
           expect(token_ids).to match_array(access_tokens.pluck(:id))
         end