Hack run_tests.sh to work with single tests again.

Even though we're moving to testr and this will eventually
go away, we should maintain our ability to run and debug
individual tests until the broader change is released.

This just adds some text around the nosetests call
to get us back where we were.

Fixes bug: 1181996

Change-Id: Id0082ca8954664679430cc602425b1fadd29151b