test suite should fail if request branch doesn't exist

[zingale]

If we ask for a source repo to be run on a branch that doesn't exist in the current git repo in the test directory (even if it exists upsteam), then we will fail silently, and the tests will be run on whatever branch was last used in that source dir.

We should instead perhaps do a

pull
checkout branch
pull

to ensure that we first see any new branches and then get to the one we want.

[drummerdoc]

Yes, I believe that is the correct procedure.