CI: replace distrotests with packaging tests using apkg

Tests in distro/pkg/deb/tests/ are symlinked into distro/tests/.

A single templated tests control file distro/tests/extra/all/control is used to describe test deps on different platforms.

Run tests with:

apkg test

See distro/tests/README.md for more info.

New CI pkg stage tests

apkg install --build-dep
apkg test --test-dep

on apkg CI images.

A single pkg:make-archive job creates archive shared in all pkg jobs.