Load test bundle earlier and run _XCTestMain later in FBXCTestMain

Summary:
Following similar things done in IDEBundleInjection (Xcode 9) and libXCTestBundleInject.dylib (Xcode 10).

We need to load test bundle earlier as it gives us flexibility in testing code to override target app's behavior before its `main` started running.

Reviewed By: superb

Differential Revision: D8526237

fbshipit-source-id: c4970f10daef190414e9befbfd8d31395a386485

Shims/Shimulator/TestLoadingShim/FBXCTestMain.m

@@ -81,6 +81,24 @@ BOOL FBXCTestMain()
     return NO;
   }
   [configuration setAbsolutePath:configurationPath];
+
+  NSURL *testBundleURL = configuration.testBundleURL;
+  if (!testBundleURL) {
+    NSLog(@"XCTestConfiguration has no test bundle URL value");
+    return NO;
+  }
+
+  NSBundle *testBundle = [NSBundle bundleWithURL:testBundleURL];
+  if (!testBundle) {
+    NSLog(@"Failed to open test bundle from %@", testBundleURL);
+    return NO;
+  }
+
+  if (![testBundle loadAndReturnError:&error]) {
+    NSLog(@"Failed load test bundle with error: %@", error);
+    return NO;
+  }
+
   void (*XCTestMain)(XCTestConfiguration *) = (void (*)(XCTestConfiguration *))FBRetrieveXCTestSymbol("_XCTestMain");
   CFRunLoopPerformBlock([NSRunLoop mainRunLoop].getCFRunLoop, kCFRunLoopCommonModes, ^{
     XCTestMain(configuration);