Cleanup shm at startup in message_bridge_test

When we start forwarding messages from before connection, we need shm to
be clean.  Take this oppertunity to dedup some things.

Change-Id: Ic4caff28ba63e48ce5e74a28868deb5c808a984d
diff --git a/aos/util/file.h b/aos/util/file.h
index bf216bb..4d0ed2d 100644
--- a/aos/util/file.h
+++ b/aos/util/file.h
@@ -26,6 +26,10 @@
 
 bool PathExists(std::string_view path);
 
+// Recursively removes everything in the provided path.  Ignores any errors it
+// runs across.
+void UnlinkRecursive(std::string_view path);
+
 }  // namespace util
 }  // namespace aos