Allow mocking of executables installed in the system

[xasopheno]

In case the script uses commands that are not installed on the system that the tests are run.

dependencies:
#       this can be install here: asdf.com
        - gcsfuse
protocols:
        - protocol: []

[soenkehahn]

Fixed through #187.

[soenkehahn]

I reverted #187, since it didn't fully work. The problem is that it didn't work on machines where executables where installed in non-standard locations, e.g. $HOME/.cargo/bin. For an example, see https://github.com/Originate/scriptkeeper/tree/sh/example.