Improve error handling in qtestlib

Dump a stack trace on Linux if the process crashes. This will give us
much better log output in the new CI system. In addition, create a
watch dog thread, that kills the test if the test function times
out (ie. hangs)

Implementations of the stack trace dumping for Mac and Windows
are still pending.

Change-Id: I65426c5afe290a0a2019b881436a0c278f1cafaf
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>