Add pretty_tox.sh wrapper script to use subunit-trace

This commit adds a pretty_tox.sh wrapper script for the tempest-lib
unit tests to use subunit-trace when running the tests. This should
make the output a bit more clear and help with future debugging.

Change-Id: I48d670353bb85f7f87a05041bed60ad131555057
Merge "Add pretty_tox.sh wrapper script to use subunit-trace"