Improves graphql_helpers

What does this MR do?

This is a CE backport of changes in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14656, with improvements to the GraphQL helper code.

Specifically this makes running tests easier, since helpers will fail sooner and with better error messages.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

this does not change any production code.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.