Try harder to find a usable lttv in the tests scripts v2

Add many more attempts to auto-detect the path to the lttv executable or
the runlttv script in trace comparison tests.
Also support setting the LTTV env variable to directly set a path the
the lttv executable in addition to the RUNLTTV variable

Changelog:
   Correctly manage multiple traces

[ Edit: fix merge conflict ]

Signed-off-by: Yannick Brosseau <yannick.brosseau@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>