drm/i915/selftests: Basic stress test for rapid context switching

We need to exercise the HW and submission paths for switching contexts
rapidly to check that features such as execlists' wa_tail are adequate.
Plus it's an interesting baseline latency metric.

v2: Check the initial request for allocation errors
v3: Use finite waits for more robust handling of broken code

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180920105809.1872-1-chris@chris-wilson.co.uk