drm/i915/selftests: Spin on all engines simultaneously

Vanshidhar Konda asked for the simplest test "to verify that the kernel
can submit and hardware can execute batch buffers on all the command
streamers in parallel." We have a number of tests in userspace that
submit load to each engine and verify that it is present, but strictly
we have no selftest to prove that the kernel can _simultaneously_
execute on all known engines. (We have tests to demonstrate that we can
submit to HW in parallel, but we don't insist that they execute in
parallel.)

v2: Improve the igt_spinner support for older gen.

Suggested-by: Vanshidhar Konda <vanshidhar.r.konda@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Vanshidhar Konda <vanshidhar.r.konda@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Vanshidhar Konda <vanshidhar.r.konda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191101101528.10553-1-chris@chris-wilson.co.uk