run_tests: add a helper for invoking unittests

This makes it very easy for people to run all our unittests with just
`./run_tests`.  There doesn't seem to be any other way currently to
quickly invoke any of the tests.

Change-Id: I1f9a3745fa397a1e797bd64065c2ba7f338de4a1
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/227613
Tested-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>