lib/core: Print thread:tid with igt_log for non-main threads

So that we know what's the source of messages.

igt_thread.c is created to facilitate more threading-related
functionality that will come in the following patch.

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