Add test coverage for development database seed fixtures

Problem to solve

The development database seed fixtures are a quite frequently recurring pain point, mainly when running gdk install. Issue #23563 (closed) 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 (closed): "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
• GDK: Schedule pipeline so database fixture errors in gitlab are automatically detected (but it would be much better to have test coverage in the gitlab repo)