igt_core: Skip sync when listing subtests

No need to sync filesystems when only listing subtest.

Extremely marginal benefit of avoid a short stall after make
followed by listing subtests.

v2: Move sync to common_init for consolidation and simplicity.
    (Chris Wilson)

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