Fix stale process after unit tests complete

After running the unit tests (eg with tox -epy27) a stale 'nosetests'
process was always left behind.  It left TCP ports open and held references on
inodes. The latter caused a problem with some automated build systems.

Rework the relevant test 'test_no_client_tracebacks' to be functionally
the same, but clean up fully.

Fixes bug 1198685.

Change-Id: If9d3afa370dc2bd3c470b509d38c94cfb5844518
Merge "Fix stale process after unit tests complete"