Fix Race in ListTestRunner
Fix Race in ListTestRunner

Summary:
This serializes all calls on the same queue otherwise this crash happens:

```
  0   CoreFoundation                      0x00007fffbd8946fb __exceptionPreprocess + 171
  1   libobjc.A.dylib                     0x00007fffd2238a2a objc_exception_throw + 48
  2   CoreFoundation                      0x00007fffbd911656 __NSFastEnumerationMutationHandler + 294
  3   CoreFoundation                      0x00007fffbd80219f -[NSArray componentsJoinedByString:] + 399
  4   fbxctest                            0x00000001092d5792 -[FBJSONTestReporter handleExternalEvent:] + 242
  5   fbxctest                            0x00000001092d9c2f __39-[FBLogicTestRunner runTestsWithError:]_block_invoke + 463
  6   fbxctest                            0x00000001093ab3a4 __38-[FBLineFileDataConsumer consumeData:]_block_invoke + 36
```

Reviewed By: nqmtuan

Differential Revision: D4231420

fbshipit-source-id: 9cff2b5c8f288e944657d33d21ebab59456ece3a

plu/FBSimulatorControl,StevenUpForever/FBSimulatorControl,plu/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,StevenUpForever/FBSimulatorControl,facebook/FBSimulatorControl,StevenUpForever/FBSimulatorControl,plu/FBSimulatorControl,plu/FBSimulatorControl,facebook/FBSimulatorControl,StevenUpForever/FBSimulatorControl,facebook/FBSimulatorControl,ichu501/FBSimulatorControl