hnfs-tester: replace the EventManager with epoll to support GHC 7.8.1

GHC 7.8.1 (base 4.7.0.0) does not support running a distinct EventManager anymore
- it can still be created, but the 'loop' call is gone.
Trying to migrate to the SystemEventManager did not work - hnfs-tester reported
spurious test failures / hangs (root cause unclear), hence the code is moved to
epoll for the time being.

Signed-off-by: Arne Redlich <arne.redlich@googlemail.com>