Don't install test files in built distributions
Right now, if you build a pmbootstrap package for distribution, it seems that all the test files get installed as part of it. This can currently be seen on pkgs.alpinelinux.org: https://pkgs.alpinelinux.org/contents?file=test_*.py&path=&name=pmbootstrap&branch=edge&repo=community&arch=x86_64
We should be able to shave off some space by not installing these given that they are unused anyway.