gh-120965: Skip test_zoneinfo.ExtensionBuiltTest if C extension is not available

[StanFromIreland]

Preserves existing behavior for tests (c_module would just be the Python implementation) other than the one failing.

• Issue: test.test_zoneinfo.test_zoneinfo.ExtensionBuiltTest.test_cache_location fails if _zoneinfo Cextension isn't built #120965

[StanFromIreland]

Requesting @pganssle since is maintainer of zoneinfo (If I recall correctly)

[pganssle]

I don't remember offhand, but I feel like the purpose of this test is to make sure nothing accidentally disables the zoneinfo C extension build, so disabling it if the C extension didn't get built seems counter-productive.

Is there a way to detect if it was intentionally not built? Something in sysconfig or something like that? Under what circumstances is it being deliberately disabled? If it is just pypy, do they have a preferred mechanism for communicating that a test like this can be skipped, preferably one that doesn't also make our tests more fragile?

[pganssle]

Oh the docstring is maybe a little misleading, I think I remember this now. It's not testing that the C extension got built, it is making sure that the zoneinfo constructor isn't returning cached values from the wrong implementation. I think. I will have to look at this a bit later to convince myself 100% that it's not also the other thing, though.