tests: fix leak when we won't run DRM-backend tests

This fixes a leak found running drm-smoke-test with ASan. Do not forget
to call prog_args_fini() when:

- skipping DRM-backend tests because we don't have
  WESTON_TEST_SUITE_DRM_DEVICE set.

- we fail to acquire the lock needed to run DRM-backend tests.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>