prefer hacks in test to hacks in code.

Adding a completion handler would be a bit gnarly since we're snaking
through a pretty long method chain.

// FREEBIE

src/Messages/OWSDisappearingMessagesJob.m

@@ -136,7 +136,7 @@ NS_ASSUME_NONNULL_BEGIN
 
 + (void)setExpirationForMessage:(TSMessage *)message
 {
-    dispatch_sync(self.serialQueue, ^{
+    dispatch_async(self.serialQueue, ^{
         [[self sharedJob] setExpirationForMessage:message];
     });
 }