test: move all tests into separate module (MR 2551)

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: #2541