[Infra] Attempt to reduce Crashlytics testing flake (#15183)

Author: ncooke3

Crashlytics/UnitTests/Mocks/FIRCLSMockFileManager.m

@@ -41,7 +41,9 @@ - (BOOL)removeItemAtPath:(NSString *)path {
   // If we set up the expectation, and we went over the expected count or removes, fulfill the
   // expectation
   if (self.removeExpectation && self.removeCount >= self.expectedRemoveCount) {
-    [self.removeExpectation fulfill];
+    dispatch_async(dispatch_get_main_queue(), ^{
+      [self.removeExpectation fulfill];
+    });
   }
 
   return YES;