test-keymap-util: always use kbd-model-map we ship

test-keymap-util: always use kbd-model-map we ship

This test makes assumptions on the availability of some mappings contained in
kbd-model-map and therefore strongly relies on the version shipped by
upstream. IOW the test is likely to fail if it's installed on a system with a
more comprehensive kbd-model-map.

This patch makes the upstream kbd-model-map file available via a symlink in
test/testdata/test-keymap-util dir and makes sure that this specific version is
always used by test-keymap-util regardless of whether the test is installed and
run on a different system or directly run (optionally via meson) from the
project working dir.