Fix breakage in tests around db extension registration.

SignalServiceKit/src/TestUtils/MockSSKEnvironment.m

@@ -131,11 +131,27 @@ NS_ASSUME_NONNULL_BEGIN
 - (void)configure
 {
     [self.databaseStorage clearGRDBStorageForTests];
+
     __block dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
     [OWSStorage registerExtensionsWithMigrationBlock:^() {
         dispatch_semaphore_signal(semaphore);
     }];
-    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
+
+    // Registering extensions is a complicated process than can move
+    // on and off the main thread.  While we wait for it to complete,
+    // we need to process the run loop so that the work on the main
+    // thread can be completed.
+    while (YES) {
+        // Wait up to 10 ms.
+        BOOL success
+            = dispatch_semaphore_wait(semaphore, dispatch_time(DISPATCH_TIME_NOW, (int64_t)(10 * NSEC_PER_MSEC))) == 0;
+        if (success) {
+            break;
+        }
+
+        // Process a single "source" (e.g. item) on the default run loop.
+        CFRunLoopRunInMode(kCFRunLoopDefaultMode, 0.0, false);
+    }
 }
 
 @end