Make it work with mocks and stubs

[Flamefire]

Fixes ztombol#8 by saving the PATH and using command to ignore functions/aliases

Note: Needed to use a subshell as PATH="$_BATSLIB_PATH" command cat - did work for the tests but failed in a real world example and I can't figure out why. Even PATH="$_BATSLIB_PATH" command -v cat outputs the correct path.

[martin-schulze-vireso]

I am closing this in favor of #9, as I can't add code here (due to the master branch?).