drm/sched: Fix a race in DRM_GPU_SCHED_STAT_NO_HANG test

drm/sched: Fix a race in DRM_GPU_SCHED_STAT_NO_HANG test

The "skip reset" test waits for the timeout handler to run for the
duration of 2 * MOCK_TIMEOUT, and because the mock scheduler opted to
remove the "skip reset" flag once it fires, this gives opportunity for the
timeout handler to run twice. Second time the job will be removed from the
mock scheduler job list and the drm_mock_sched_advance() call in the test
will fail.

Fix it by making the "don't reset" flag persist for the lifetime of the
job and add a new flag to verify that the code path had executed as
expected.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Fixes: 1472e7549f84 ("drm/sched: Add new test for DRM_GPU_SCHED_STAT_NO_HANG")
Cc: Maíra Canal <mcanal@igalia.com>
Cc: Philipp Stanner <phasta@kernel.org>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://lore.kernel.org/r/20250716084817.56797-1-tvrtko.ursulin@igalia.com