Integrate downstream Debian autopkgtests via DESTDIR staging

[rkt0209]

Summary

Wires the downstream Debian autopkgtests into the existing CI so they run
against a staged build tree - unprivileged, with no mount --bind, proot,
or privileged containers.

What's included

• DESTDIR staging (make stage-ciroot): installs into an ephemeral
  _ciroot/, stages the 8 make check test binaries and their image
  data, and rewrites libcupsfilters.pc to point at the staged tree.
• Both downstream autopkgtests wired into the build on every arch:
  • libcupsfilters-2-dev — pkg-config compile/link/run under
    dbus-run-session.
  • libcupsfilters-2-functionality — runs all 8 test programs
    (testdither, testpdf1/2, test-analyze, test-pdf, test-ps,
    testcmyk, testrgb) against the staged binaries and data.

Approach

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