TestUserDefinedCounters.testMapReduceJob is flakey

TestUserDefinedCounters.testMapReduceJob is flakey.

We sometimes see it fail:

junit.framework.AssertionFailedError
	at junit.framework.Assert.fail(Assert.java:48)
	at junit.framework.Assert.assertTrue(Assert.java:20)
	at junit.framework.Assert.assertTrue(Assert.java:27)
	at org.apache.hadoop.mapred.TestUserDefinedCounters.testMapReduceJob(TestUserDefinedCounters.java:113)

Upon investigation, the problem is that the input for the MR job in this test is at System.getProperty("test.build.data", "/tmp") + "/input". If an earlier test wrote some files there, this test will use them as part of its input. This can cause all sorts of problems with this test because its not expecting the additional input data.