tests: Fix seg fault when gem_mmap is run without specifying a subtest

tests: Fix seg fault when gem_mmap is run without specifying a subtest

gem_mmap seg faults when all tests are run together. This occurs because
the new-object subtest closes the gem object, but short-mmap assumes
it still exists. Thus gem_mmap__cpu() returns nil for addr and memset()
seg faults. This patch makes new-object and short-mmap create and
close their own gem objects.

Signed-off-by: Mike Mason <michael.w.mason@intel.com>
Signed-off-by: Thomas Wood <thomas.wood@intel.com>