Add rmdir to testlib
CA-134784: cover refcounter with unit tests
CA-134784: Fix race conditions in refcounter

Concurrent refcounter operations on two different refcounters sharing
the same namespace can lead to race conditions:
 A.) By the time the refcounter file is written, the namespace directory
     has already been removed
 B.) Removing a namespace directory fails, because a a new refcounter file
     has been created within the same directory

_writeCount now returns False if the file was not found

A is solved by spinning on _writeCount until it succeeds.
B is solved by ignoring the errors

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>