Make testing with the multi-binary platform independent

[syrmel]

Up to now, the testing logic was that if we test in Windows, the multi-binary gets used, otherwise it's the individual command line utils. Disentangling platform and multi-binary use creates flexibility to the future, such as:

• set up tests with the multi-binary on *nix
• use other versions of Windows coreutils (like GNU instead of the current Rust version from uutils) to test against

... and the code is a little cleaner with fewer $if !windows :)