tests/gem_(cpu|gtt)_concurrent_blit: Enable signals

tests/gem_(cpu|gtt)_concurrent_blit: Enable signals

In order to exercise the bug behind:

commit 22fd5ca947b58901927d100d2b1aa0f1672b3435
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Jun 28 16:54:08 2013 +0100

    drm/i915: Only clear write-domains after a successful wait-seqno

we need to check for concurrent access with the potential to be
interrupted by a signal. The framework for doing so is already in place,
so just enable it and repeat the tests for longer to give better chance
of being interrupted at just the wrong moment.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>