edtlib: warn on non-tokenizable enums

These are likely enough defined by mistake to emit a warning for.

Adjust tests to match, tweaking the test_warnings() setup: now that
we've got several test cases, it's a bit cleaner not to have to
copy/paste the ('edtlib', WARNING, ...) part of every expected log
record tuple.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>