Add always_fail_test

Add a test that always fails to make sure that the pass/fail logic is
working.
Convert 'fail' to a 'exit(fail)'

A wrapper must be used to convert a test's return to an exit code.
Otherwise the returned term is ignored and tests that return 'fail'
are treated as passing.
Merge pull request #613 from basho/rz/no-fails

A return of `fail` should actually report failure