RFR 8205547 : FileChannel/CleanerTest.java fails due to expected FD count

[Roger Riggs]

Please review a test improvement to avoid unexpected file opens/closes 
during the file channel CleanerTest.
The test now follows the pattern of the other 
java/io/File*/UnreferencedXXXClosesFd tests by waiting
for the cleaner and file descriptors to be reclaimed by GC.
Webrev:
  http://cr.openjdk.java.net/~rriggs/webrev-fd-count-wrong-8205547/

Thanks, Roger