Flaky test for open file descriptors

In random situations (like here https://travis-ci.org/apache/avro/jobs/548635040 ) the test TestDataFileReader : testForLeakingFileDescriptors fails with a message like this
 

[ERROR] testForLeakingFileDescriptors(org.apache.avro.TestDataFileReader) Time elapsed: 0.026 s <<< FAILURE!
java.lang.AssertionError: File descriptor leaked from new DataFileReader() expected:<38> but was:<35>

The assessment is that during the test run the garbage collector cleaned up some of the open file descriptors and thus the expected number of open file descriptors is lower than expected by the test.