Autopkgtest task should be able to run autopkgtest from different releases

Currently the autopkgtest task runs autopkgtest as it is installed in the worker: i.e. it usually uses a version of autopkgtest in a recent Debian release.

The "environment" parameter only controls the environment fed to autopkgtest to execute the tests, and not the environment where autopkgtest is actually run.

Keeping the support for very old Debian releases has a cost for autopkgtest's upstream maintainers as some of its code which runs inside the testbed can't use newer features (cf upstream README.md).

This issue is about improving the autopkgtest task so that it becomes possible to run older version of autopkgtest when testing packages on older Debian/Ubuntu releases (so that newer autopkgtest releases can more aggressively drop compatibility support for older Debian/Ubuntu releases).

This was requested by @elbrus in #debci.

Initial description by Jochen

autopkgtest depends on features in the host and guest system making it behave different for different versions of Debian. The autopkgtest team would like to drop old features and switch to newer functions meaning that newer autopkgtest versions would no longer support old Debian versions. As debusine should support old Debian versions, it should run autopkgtests also with the autopkgtest release suitable for that Debian version.