config: add an integration test

Add a test beyond unittests to help catch real world config logic.
This would have caught the regression in placeholder expansion.

This actually caught REPO_PATH being set incorrectly.  But doesn't
seem like anyone is using it currently, so not a big deal.

Bug: 139175041
Test: repo upload works
Change-Id: I7f6a10b3bceffcfcddeb26aa91a2048ca3d52aeb