Add tests for timezone aware ExtensionArrays

[dcherian]

What is your issue?

We have support for timezones now (suprise!) thanks to ExtensionArray support.

A brief non-exhaustive list of what might be needed to build some confidence here:

• we should add such an array to create_test_dataset so we have some minimal testing.
• we'll need to build DatetimeCoder support: Error saving xr.Dataset with timezone aware time index to netcdf format. #3320
• isel still seems to be broken: Calling .isel() on a timezone-aware dimension/index causes it to lose timezone information #9307