drm/i915/selftests: Exercise reset to break stuck GTT eviction

We must be able to reset the GPU while we are waiting on it to perform
an eviction (unbinding an active vma). So attach a spinning request to a
target vma and try and it evict it from a thread to see if that blocks
indefinitely.

v2: Add a wait for the thread to start just in case that takes more than
10ms...
v3: complete() not completion_done() to signal the completion.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180716134009.13143-1-chris@chris-wilson.co.uk