Backend-level test coverage for remaining 10 backends

[trmxvibs]

tests/backends/test_apt_backend.py is the only backend with dedicated
unit tests right now. The same mocking pattern should be applied to the
other 10 backends (pacman, dnf, apk, brew, zypper, xbps, nix, eopkg, freebsd, pkg/Termux) to catch regressions early.