When installing pmboostrap, all tests are installed alongside it. There doesn't seem to be any way to selectively exclude some files inside python modules, so move all test into a separate module instead. This is the typical convention in python projects. See: https://gitlab.alpinelinux.org/alpine/aports/-/merge_requests/79862 Fixes: https://gitlab.postmarketos.org/postmarketOS/pmbootstrap/-/issues/2541
Hopefully makes for easier to read code, and potentially also faster once we have mypyc and the enum can get compiled into plain integers instead of the strings we previously were working with.
