Workhorse route specific tests for Geo secondary proxying

The following discussion from !71732 (merged) should be addressed:

• @cat started a discussion: (+1 comment)

  This is really only testing the geoGitProjectPattern in !71732 (diffs, comment 696487891), but it only makes sense when proxying is enabled, on a secondary (so didn't add the equivalent route to the other testcases, because "it should technically not be called in the other cases") and it's caught by the "unknown route" test anyway, but happy to add it explicitly if this feels worth it.

  @mkozono:

  If we want to cover every interesting route, maybe we should add a routes_test.go. Do you think it's worth opening a follow up?