drm/i915/selftests: Include the start of each subtest in the GEM trace

Knowing the boundary of each subtest can be instrumental in digesting
the voluminous trace output and finding the critical piece of
information.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180713203529.1973-1-chris@chris-wilson.co.uk