Don’t dispatch for non-specta tests

The swizzled functionality is causing deadlocks when an XCTestExpectation times out in a project that includes Specta.

Specta/Specta/XCTestCase+Specta.m

@@ -42,7 +42,10 @@ - (void)spt_dequeueFailures {
     [self spt_dequeueFailures];
   };
 
-  if ([NSThread isMainThread]) {
+  BOOL isMainThread = [NSThread isMainThread];
+  BOOL isSpectaTest = [self isKindOfClass:[SPTSpec class]];
+
+  if (!isSpectaTest || isMainThread) {
     dequeueFailures();
   } else {
     dispatch_sync(dispatch_get_main_queue(), dequeueFailures);