Tests fail on Darwin (macOS) when GNU coreutils are on the path

[therealpxc]

The tests for this program assume that running Darwin means that FreeBSD's coreutils are on the path, but that's not necessarily the case. With the Nix package manager, this breaks builds of tldr because the GNU coreutils are part of the build environment, even on Darwin. This issue will also affect users who have installed the GNU coreutils through Homebrew or some other package manager.

Since running macOS (or *BSD) is actually a distinct proposition from using non-GNU coreutils, the tests should probably reflect that.

[psibi]

Sorry for my late reply. I would welcome a PR fixing this issue.