Fix the :project factory by not copying the test repo twice

What does this MR do?

This fixes the :project factory by removing a call to TestEnv.copy_repo which is already done by the :test_repo trait, included in this factory.

This also replaces calls to create(:project, :test_repo) by just create(:project) since the :project factory already includes the :test_repo trait.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Because our test suite could run much faster (and will)!

What are the relevant issue numbers?

I discovered this while reviewing @rspeicher benchmarks in #24899 (moved) (thanks @rspeicher!).