autopkgtest: figure out which packages from unstable are needed

Currently autopkgtest tries to install our trigger from unstable and the rest
from testing. If that fails, than autopkgtest has a fall-back to allow all
packages from unstable to be installed. This has two severe issues:

1) the version of the test and the package it came from may be out-of-sync

2) too much from unstable may be installed, even stuff that should not/is not
allowed to migrate as it breaks stuff.

Make sure that test depends also get added to triggers if they are broken.
E.g. imagine the following scenario: trigger X changes (breaks) the output
generated by Y. Package Z has Y in the test dependencies and compares the
output in the autopkgtest. We want to have the opportunity that a new version
is automatically fixing the situation.

Two use cases are currently unsupported: needs-build (autopkgtest restriction)
and test dependencies generated by autodep8.