gem_storedw_loop: fix segfault when listing subtests

Commit fb66cd5 (tests/gem_storedw_loop: Fix use after free for bufmgr)
introduced a segmentation fault when listing subtests because
drm_intel_bufmgr_destroy is called with NULL. Move this and the call to
the close function inside an igt_fixture block to prevent them being
called when listing subtests.

Cc: Robert Beckett <robert.beckett@intel.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thomas Wood <thomas.wood@intel.com>

tests/gem_storedw_loop.c

@@ -180,6 +180,8 @@ igt_main
 		}
 	}
 
-	drm_intel_bufmgr_destroy(bufmgr);
-	close(fd);
+	igt_fixture {
+		drm_intel_bufmgr_destroy(bufmgr);
+		close(fd);
+	}
 }