Fix the failing test.

Author: visumickey

FirebasePerformance/Tests/Unit/FPRTraceBackgroundActivityTrackerTest.m

@@ -38,13 +38,25 @@ - (void)testForegroundTracking {
 
 /** Validates if the foreground & background state is captured correctly. */
 - (void)testBackgroundTracking {
+  XCTestExpectation *expectation = [self expectationWithDescription:@"Application state change"];
+  
   FPRTraceBackgroundActivityTracker *tracker = [[FPRTraceBackgroundActivityTracker alloc] init];
   NSNotificationCenter *defaultCenter = [NSNotificationCenter defaultCenter];
-  [defaultCenter postNotificationName:UIApplicationDidBecomeActiveNotification
-                               object:[UIApplication sharedApplication]];
-  [defaultCenter postNotificationName:UIApplicationDidEnterBackgroundNotification
-                               object:[UIApplication sharedApplication]];
-  XCTAssertEqual(tracker.traceBackgroundState, FPRTraceStateBackgroundAndForeground);
+  dispatch_async(dispatch_get_main_queue(), ^{
+    [defaultCenter postNotificationName:UIApplicationDidBecomeActiveNotification
+                                 object:[UIApplication sharedApplication]];
+    [defaultCenter postNotificationName:UIApplicationDidEnterBackgroundNotification
+                                 object:[UIApplication sharedApplication]];
+    [expectation fulfill];
+  });
+  
+  [self waitForExpectationsWithTimeout:5.0 handler:^(NSError * _Nullable error) {
+    if (error) {
+        XCTFail(@"Expectation failed with error: %@", error);
+    } else {
+      XCTAssertEqual(tracker.traceBackgroundState, FPRTraceStateBackgroundAndForeground);
+    }
+  }];
 }
 
 @end