[TestNG] Don't fail if TestNG runs in parallel

Created by: navkast

For TestNG, a @Test method can run concurrently using a @DataProvider:

@DataProvider(name = "testList", parallel = true)

This causes an exception in Buck as follows:

java.util.ConcurrentModificationException
	at java.base/java.util.ArrayList$Itr.checkForComodification(ArrayList.java:1042)
	at java.base/java.util.ArrayList$Itr.next(ArrayList.java:996)
	at com.facebook.buck.testrunner.BaseRunner.writeResult(BaseRunner.java:100)
	at com.facebook.buck.testrunner.TestNGRunner.run(TestNGRunner.java:96)
	at com.facebook.buck.testrunner.BaseRunner.runAndExit(BaseRunner.java:301)
	at com.facebook.buck.testrunner.TestNGMain.main(TestNGMain.java:48)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at com.facebook.buck.jvm.java.runner.FileClassPathRunner.main(FileClassPathRunner.java:88)

The fix is to simply move the test runner's collection of test results to be a concurrency-safe collection. I use ConcurrentLinkedQueue: this should be faster than explicit synchronized blocks or using Collections.synchronizedList().