drm/i915/selftests: Rename wait_for_hang() to wait_until_running()

Tvrtko mentioned that wait_for_hang() was confusing as it does not
actually wait for the aforementioned hang, just until the request is
running and we are *ready* to inject a hang. A quick
s/wait_for_hang/wait_until_running/ removes that confusion without
having to rethink the naming scheme, immediately at least.

Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180406100950.19033-1-chris@chris-wilson.co.uk