`rake clobber` should use `bundle exec`.
Base host on the currently configured address, don't hardcode "localhost".
Only ignore FLUSHALLDATA system test when testing against simulator.

(It's a no-op in the simulator, so testing whether asynchronous PUTs
and DELETEs are successfully completed by a FLUSHALLDATA will never
give accurate info.)
Eliminate unnecessary 'return's in tail calls.