Fixes#3244.
The ui-egui tests look up English labels, but Session::default() keeps the
language pref "system", so on a German (or any non-English) system they
read translated labels and fail. Match System now resolves to English under
cfg(test) (lib tests) and under a new test-only `test-support` feature,
which the crate's own dev-dependency on itself turns on for tests/it,
examples and workspace test builds. Product builds are unchanged; tests
about a translation still set their language explicitly. The Match System
test now asserts the English pin explicitly.
vangel76
c54b542274