Bug 1600130 - Sync HTTP fuzzing target with background thread. r=mayhemer

Differential Revision: https://phabricator.services.mozilla.com/D56694

--HG--
extra : moz-landing-system : lando

netwerk/test/fuzz/TestHttpFuzzing.cpp

@@ -218,6 +218,16 @@ static int FuzzingRunNetworkHttp(const uint8_t* data, size_t size) {
     // Wait for StopRequest
     gStreamListener->waitUntilDone();
 
+    bool mainPingBack = false;
+
+    NS_DispatchBackgroundTask(NS_NewRunnableFunction("Dummy", [&]() {
+      NS_DispatchToMainThread(NS_NewRunnableFunction("Dummy", [&]() {
+        mainPingBack = true;
+      }));
+    }));
+
+    SpinEventLoopUntil([&]() -> bool { return mainPingBack; });
+
     channelRef = do_GetWeakReference(gHttpChannel);
   }