lib/core: Handle asserts in threads

Since IGT is using magic control blocks (igt_subtest et al.) asserts
jump out the them using longjmp() causing a bunch of confusing messages
and occasionally crashing the whole process.

This is not the behavior we want :-)

With this patch:

1. simple_main, dynamic and subtest each clears the thread failure state
   at the start

2. each of those blocks also asserts no thread failures at the end

3. igt_assert() in non-main thread prints out the error + stacktrace
   and marks that we have failed thread

Issue: #55


Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Petri Latvala <petri.latvala@intel.com>