Fix false positives in graphql tests

What does this MR do and why?

Fixes https://gitlab.com/gitlab-org/gitlab/-/issues/342383

Some tests didn't really test anything because the changes weren't persisted to the ActiveRecords model.

Screenshots or screen recordings

Only tests are modified

How to set up and validate locally

Run the specs. There's probably a better way to do this with an rspec CLI option but I used bundle exec spring rspec $(rg -l 'permission level for .* mutation is correctly verified') (rg is ripgrep)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.