Move as many timezone tests as possible to be provider-agnostic

[Manishearth]

Currently we have tests for timezone stuff in a couple places: builtins/compiled/zoneddatetime.rs, builtins/core/timezone.rs, and tzdb.rs.

They all test compiled data.

@robertbastian is working towards zoneinfo64 (and I know @nekevss was also working on this problem e.g. in #257). It would be nice if all of these tests could run in that configuration as well.

We could probably have a testing only get_testing_provider() function that exists on compiled data OR zoneinfo data, and makes a provider based on the feature set being test with. All of these tests should then use it.

Filing now mostly to keep track of the scattered tests.