chore: skip orc test if pyarrow.orc is not available

[tswast]

The pyarrow.orc module is an optional extension that isn't available internally.

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Fixes #<issue_number_goes_here> 🦕

[chelsea-lin]

LGTM. I am wondering how the test passes before this change without checking the package?

[tswast]

LGTM. I am wondering how the test passes before this change without checking the package?

Before my cl/788930779, we were skipping this test internally. I think it's pretty rare to get a custom pyarrow that has some C++ modules excluded "in the wild", so I'm not too surprised that we didn't see this before.