drm/i915/selftests: Fix up igt_reset_engine

Now that we skip a per-engine reset on an idle engine, we need to update
the selftest to take that into account. In the process, we find that we
were not stressing the per-engine reset very hard, so add those missing
active resets.

v2: Actually test i915_reset_engine() by loading it with requests.

Fixes: f6ba181 ("drm/i915: Skip an engine reset if it recovered before our preparations")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104313
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171217132852.30642-3-chris@chris-wilson.co.uk
Reviewed-by: Michel Thierry <michel.thierry@intel.com>