test-gvfs-protocol: add cache_http_503 to mayhem

Create new `cache_http_503` mayhem method where only the cache server
sends a 503.  The normal `http_503` directs both cache and origin
server to send 503s.  This will be used to help test fallback.

Signed-off-by: Jeff Hostetler <jeffhostetler@github.com>

Author: jeffhostetler

Diff for: t/helper/test-gvfs-protocol.c

@@ -1581,6 +1581,14 @@ static enum worker_result dispatch(struct req *req)
 	const char *method;
 	enum worker_result wr;
 
+	if (strstr(req->uri_base.buf, MY_SERVER_TYPE__CACHE)) {
+		if (string_list_has_string(&mayhem_list, "cache_http_503")) {
+			logmayhem("cache_http_503");
+			return send_http_error(1, 503, "Service Unavailable", 2,
+					       WR_MAYHEM | WR_HANGUP);
+		}
+	}
+
 	if (string_list_has_string(&mayhem_list, "close_no_write")) {
 		logmayhem("close_no_write");
 		return WR_MAYHEM | WR_HANGUP;