In Plurals and other components, don't emit non-modern locales #422
Description
We should thoroughly test behavior in modern locales. This should be sufficient for testing conformance with the spec.
Implementations like ICU4X don't ship non-modern locales by default. This results in test failures being observed due to the behavior falling back to the root locale. Example: #419
Rather than making ICU4X mark these as unsupported, the conformance suite should be omitting these from the test data.