drm/i915/selftests: Trim execlists runtime

Reduce the smoke depth by trimming the number of contexts, repetitions
and wait times. This is in preparation for a less greedy scheduler that
tries to be fair across contexts, resulting in a great many more context
switches. A thousand context switches may be 50-100ms, causing us to
timeout as the HW is not fast enough to complete the deep smoketests.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200607222108.14401-5-chris@chris-wilson.co.uk