drm/i915/selftests: Exercise context switching in parallel

We currently test context switching on each engine as a basic stress
test (just verifying that nothing explodes if we execute 2 requests from
different contexts sequentially). What we have not tested is what
happens if we try and do so on all available engines simultaneously,
putting our SW and the HW under the maximal stress.

v2: Clone the set of engines from the first context into the secondary
contexts.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190930144919.27992-1-chris@chris-wilson.co.uk