git: Move all remaining tests out of vcs/git

Created by: ryanslade

Move all remaining tests from internal/vcs/git into internal/gitserver or internal/gitserver/integration_tests.

Tests that still require a running gitserver instance were moved into internal/gitserver/integration_tests

Closes https://github.com/sourcegraph/sourcegraph/issues/33280

Huge thanks to @mollylogue @sashaostrikov and @vrto for all your help getting this over the line 🥳

Test plan

All tests still pass