Allow pdb debugging in manually-invoked tests.

Allow pdb debugging in manually-invoked tests.

 * The old behavior required setting environment variables to
   disable capture of stdout and stderr to allow debugging via pdb.
   The new behavior requires enabling capture of stdout/stderr
   instead. This is already the case in nova.
 * testr behavior is unmodified.
 * Tip 'o the hat to Robert Collins (lifeless)

Change-Id: Ica021eeab07d5a540ce42436c9cc8000c3fd239a