tests: verify that fts diagnoses readdir() failures

tests: verify that fts diagnoses readdir() failures

* tests/rm/rm-readdir-fail.sh: A new test to simulate readdir()
failing immediately or after returning a few entries, and verifying
that rm does the appropriate thing.
This was initially reported at:
http://bugzilla.opensuse.org/show_bug.cgi?id=984910
where it was mentioned that readdir() may fail
when an NFS server has a poor readdir cookie implementation.