Harness: Ensure $DONE called only once in async-gc.js

This looks like a bug: resolveAsyncGC() is supposed to succeed if the
thrown value is asyncGC.notCollected, but instead it would call $DONE()
twice. An added "return" prevents that.

harness/async-gc.js

@@ -50,6 +50,7 @@ function resolveAsyncGC(err) {
   if (err === asyncGC.notCollected) {
     // Do not fail as GC can't provide necessary resources.
     $DONE();
+    return;
   }
 
   $DONE(err);