tests/gem_exec_big: Re-add gem_sync

tests/gem_exec_big: Re-add gem_sync

We need this to avoid hitting the slowpath and ending up with a
presumed_offset == -1. Regression reported by PRTS, bisected to

commit eb36fc993d7ae1988c80ba5b767989059c91d0ec
Author:     Chris Wilson <chris@chris-wilson.co.uk>
AuthorDate: Mon Jun 16 10:49:16 2014 +0100
Commit:     Chris Wilson <chris@chris-wilson.co.uk>
CommitDate: Mon Jun 16 10:51:02 2014 +0100

    igt/gem_exec_big: Update to new igt_assert_eq

    Use igt_assert_eq for better test output on failures.

    Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
v2: igt_warn_on unexpected reloc offsets.

Cc: shuang.he@intel.com
Acked-by: Chris Wilson <chris@chris-wilson.co.uk> (on irc)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>