test: Fix memory leak in unit test run-gossmap_local

This was likely missed because we don't run the tests under valgrind anymore
due to time constraints. I do run them on a semi-regular basis, which is why
I found this.

common/test/run-gossmap_local.c

@@ -436,4 +436,5 @@ int main(int argc, char *argv[])
 	/* Now we can refresh. */
 	assert(write(fd, "", 1) == 1);
 	gossmap_refresh(map);
+	common_shutdown();
 }