declare original location of cmds

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 barcoded
`/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()`.