autopkgtest: doesn't install dependencies from experimental or backports

Similar to #58, if the package under test is built for experimental and has dependencies that cannot be satisfied in unstable, or if it is build for $foo-backports and has dependencies that cannot be satisfied in $foo (where $foo is buster, stretch etc.), then autopkgtest fails to install test-dependencies. This is because experimental and -backports are pinned to a lower priority than their base suites, and apt does not consider installing a lower-priority package if the candidate (highest priority) version is not enough.

aptitude does consider installing lower-priority packages, which is why it's used for the build-dependencies in the build stage (and for official -backports buildds), but autopkgtest doesn't have an option to use aptitude for package installation.

It might be possible to fix this by dropping an apt.conf snippet into the autopkgtest container, to make apt use apt-cudf with the aspcud solver (as used for official experimental buildds), which would also consider lower-priority packages. I'll try implementing this at some point.

A workaround is to skip the autopkgtest stage whenever one of these overlay suites is targeted:

autopkgtest:
  # https://salsa.debian.org/salsa-ci-team/pipeline/issues/85
  except:
    variables:
      - $RELEASE == "experimental"
      - $RELEASE =~ /-backports$/