drm/i915/selftest: Analyse timestamp behaviour across context switches

Check that the CTX_TIMESTAMP is monotonic across context save/restore
and upon preemption.

References: https://gitlab.freedesktop.org/drm/intel/issues/1233


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200219112004.1412791-1-chris@chris-wilson.co.uk