Minor Description
TestTaskAttemptContainerRequest should reset UserGroupInformation after end since this test cache UserGroupInformation and can be cause of other test fail. For example, all test in TestRMContainerAllocator will be failed if we run these tests after TestTaskAttemptContainerRequest (or we can change "UserGroupInformation.setLoginUser(null)" with UserGroupInformation.reset() in this class).
Also, I think the WARNING message in TestTaskAttemptContainerRequest can be removed since we can reset UserGroupInformation after each test.
