Report stdout from test listing on failure
Report stdout from test listing on failure

stdout from `testr list-tests' often contains useful information, so
report it to the user on failure.

Change-Id: Idc9d2e16d64fcdfad1de881a3458d13a7de33471
Closes-Bug: #1533655

stackforge/refstack-client,openstack/refstack-client,openstack/refstack-client,stackforge/refstack-client