Run downstream Debian autopkgtests in CI via DESTDIR staging

[rkt0209]

Summary

Adds the downstream Debian autopkgtests to CI, run against a DESTDIR-staged
build tree — unprivileged, with no mount --bind, proot, or privileged
containers. The tests resolve all paths through environment overrides, so the
same suite runs identically on native and QEMU-emulated architectures.

What's included

• make stage-ciroot — installs into an ephemeral _ciroot/ via
  make install DESTDIR=…, stages the make check test binaries and their
  image data into it, and rewrites libcupsfilters.pc to point at the staged
  tree.
• ci/autopkgtest/run.sh — a universal wrapper that points
  PATH / LD_LIBRARY_PATH / PKG_CONFIG_PATH at the staged tree and runs
  the vendored downstream test scripts.
• Both downstream tests, vendored under ci/autopkgtest/debian-tests/:
  • libcupsfilters-2-dev — pkg-config compile/link/run of a consumer program
    against the staged library, under dbus-run-session.
  • libcupsfilters-2-functionality — runs the make check test programs
    (testdither, testpdf1/2, test-analyze, test-pdf, test-ps,
    testcmyk, testrgb) against the staged binaries and data.
• make test-autopkgtest target wiring it together, invoked from the CI
  workflow on every architecture.

Approach

The downstream scripts default to the system /usr paths but accept
environment overrides (LIBCUPSFILTERS_BINDIR, LIBCUPSFILTERS_TESTDIR) that
redirect them into the staging tree. The only change to the downstream
libcupsfilters-2-functionality script is two : "${VAR:=/usr/...}" fallback
lines, so it stays fully backward-compatible with the normal Debian
autopkgtest run.