tests/perf_pmu: Verify engine busyness accuracy

A subtest to verify that the engine busyness is reported with expected
accuracy on platforms where the feature is available.

We test three patterns: 2%, 50% and 98% load per engine.

v2:
 * Use spin batch instead of nop calibration.
 * Various tweaks.

v3:
 * Change loops to be time based.
 * Use __igt_spin_batch_new inside timing sensitive loops.
 * Fixed PWM sleep handling.

v4:
 * Use restarting spin batch.
 * Calibrate more carefully by looking at the real PWM loop.

v5:
 * Made standalone.
 * Better info messages.
 * Tweak sleep compensation.

v6:
 * Some final tweaks. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

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