#794 MRUListTest breaks headless CI (c-tests-sanitizers, c-tests-valgrind) on GTK3V2MAIN

[Allen D Byrne]

SF r7309 (Merge bug-mrulist-orphan-duplicate-gtk3 into GTK3V2MAIN, Martin Fischer) added app/wlib/gtk3lib/unittests/mrulisttest.c, registered as ctest 'MRUListTest'. Like the existing PreferenceTest, it opens a real GTK3 display (GtkWindow etc.) and aborts with 'Gtk-WARNING: cannot open display' when none is available.

Two GitHub Actions jobs in .github/workflows/ci-gtk3.yml run ctest directly (no xvfb-run) and already exclude PreferenceTest by name for this exact reason: c-tests-sanitizers and c-tests-valgrind. Neither excludes MRUListTest yet, so both fail on any branch/PR carrying this new test -- confirmed on git PR #197 (upstream sync of bug-mrulist-orphan-duplicate-gtk3), run 34866023869.

Fix: add MRUListTest alongside PreferenceTest in both jobs' ctest -E exclusion (same pattern, same rationale -- covered elsewhere by c-tests-linux/regression-gtk3-sanitizers under xvfb-run).

[Allen D Byrne]

Fixed: added MRUListTest alongside PreferenceTest in both c-tests-sanitizers and c-tests-valgrind's ctest -E exclusion (ci-gtk3.yml), since it needs a real GTK3 display like PreferenceTest already does. Also updated docs/doxygen/advanced.md and building.md, which document the PreferenceTest exclusion this extends.

Verified via GitHub CI on git PRs #197 (repro), #199, #200 (all green), and locally: DISPLAY= ctest -R MRUListTest fails with the same Gtk-WARNING the CI jobs hit; the -E exclusion fixes it; full 17-test suite passes with a real display.

Merged into git GTK3V2MAIN (011acd2b) and Hg GTK3V2MAIN mainline (r7322, bug-794-mrulisttest-ci-headless-gtk3), pushed to SF.