Add test coverage for development database seed fixtures
### Problem to solve
The [development database seed fixtures](https://gitlab.com/gitlab-org/gitlab/tree/master/db/fixtures/development) [are a](https://gitlab.com/gitlab-org/gitlab/issues/32393) [quite](https://gitlab.com/gitlab-org/gitlab/issues/36876) [frequently](https://gitlab.com/gitlab-org/gitlab/issues/36919) [recurring](https://gitlab.com/gitlab-org/gitlab-development-kit/issues/424) [pain](https://gitlab.com/gitlab-org/gitlab-development-kit/issues/578) [point](https://gitlab.com/gitlab-org/gitlab-development-kit/issues/594), mainly when running `gdk install`. Issue #23563 summarizes the problem: "We had a lot of issues in the past with seeds, and I often heard that seeds were broken. ... We don't really test the seeds beside generating them in the CI." That was supported in #36876: "We should probably add a separate CI job that runs our seeds".
### Proposal
Add test coverage for the seed fixtures.
### Concerns
This test coverage could be quite slow to execute because these seed fixtures deliberately generate more than minimal database records.
### What does success look like, and how can we measure that?
Success would be at least an 80% reduction in bugs involving the seed fixtures.
### Links / references
- [Revamp the dev seed](https://gitlab.com/gitlab-org/gitlab/issues/23563)
- GDK: [Schedule pipeline so database fixture errors in gitlab are automatically detected](https://gitlab.com/gitlab-org/gitlab-development-kit/issues/611) (but it would be much better to have test coverage in the gitlab repo)
issue