The previous tests assumed default locations for all commands, however
they do not exist in the same place on all *nixes. I encountered this
when the test was failing on MacOSX where it was expected hardcoded
`/bin/sed` when in fact the code was identifying the correct local
expansion of `/usr/bin/sed`.
To make this test more robust, it now checks for the original location
of all cmds and uses that in its expansion tests for
`test_shell_command_wrapping()`.
ae4b93f52f