On other systems, we expected to find system Ruby in `/usr/bin`, but in
OpenBSD 5.4 it will be found in `/usr/local/bin`.
This replaces the limited USRBIN_ALT hack with a more generic
`path_without` function that will ensure that the given executable is
not present in the resulting PATH even if it's found in multiple
system paths.
The error was "bash: no such file or directory" and it was due to bash
being located in `/usr/local/bin` on OpenBSD 5.4 instead of `/bin` like
on other systems.
Fixed by keeping `/usr/local/bin` in PATH during the test run.
