test-keymap-util: use kbd-model-map/language-fallback-map from $(srcdir)

test-keymap-util: use kbd-model-map/language-fallback-map from $(srcdir)

This adds (undocumented) environment variables SYSTEMD_KBD_MODEL_MAP
and SYSTEMD_LANGUAGE_FALLBACK_MAP, which, if set, override compiled-in
locations of those two files.

Instead of skipping tests when the maps are not installed, just use
the one from the source dir. We still cannot do the mappings the other
way if /usr/lib/kbd/keymaps is not present, so truncate the tests in
that case.

Also tweak the debug messages a bit to make it easier to see
which function is failing.