tests: Use igt_exit for tests with subtest

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

tests/debugfs_pipe_crc.c

@@ -268,5 +268,5 @@ int main(int argc, char **argv)
 		fclose(data.ctl);
 	}
 
-	return 0;
+	igt_exit();
 }

tests/gem_mmap.c

@@ -97,5 +97,5 @@ int main(int argc, char **argv)
 	igt_fixture
 		close(fd);
 
-	return 0;
+	igt_exit();
 }

tests/gem_storedw_batches_loop.c

@@ -167,5 +167,5 @@ int main(int argc, char **argv)
 		close(fd);
 	}
 
-	return 0;
+	igt_exit();
 }