The test fails because of a race condition between the test and the replication monitor. The default replication monitor interval is 3 seconds, which is just about how long the test normally takes to run. The test deletes a file and then subsequently gets the namesystem writelock. However, if the replication monitor fires in between those two instructions, the test will fail as it will itself invalidate one of the blocks. This can be easily reproduced by removing the sleep() in the ReplicationMonitor's run() method in BlockManager.java, so that the replication monitor executes as quickly as possible and exacerbates the race.
To fix the test all that needs to be done is to turn off the replication monitor.