fuse: provide a stub "flush" implementation (RHBZ#660687).

It seems that FUSE can invoke flush to make sure the pending changes
(e.g. to the attributes) of a file are set. Since a missing flush
implementation is handled as if it were returning ENOSYS, this can cause
issues later.

To overcome this, just provide a stub implementation which does nothing,
since we have nothing to do and don't want to have FUSE error out.

Furthermore, uncomment the timestamp checks in test-fuse.sh, since now
they should be working fine.