Add a TESTDIRS variable to the test bundle to allow for the running of a
single go directory worth of tests.

Docker-DCO-1.1-Signed-off-by: Victor Marmol <vmarmol@google.com> (github: vmarmol)
Merge pull request #5477 from vmarmol/add-testdirs

Add a TESTDIRS variable to the test bundle to control which directories are unit tested