[CI] Add XCTests to GitHub Actions CI

Summary:
Self-explanatory, these tests weren't being run on CI until now.

Depends on D4622 (even though it has higher diff ID, sorry about that)

Test Plan: Worked locally... will see whether CI job succeeds after landing (should be quick revert if there are issues).

Reviewers: tomek, jacek, karol, abosh, varun

Reviewed By: karol, varun

Subscribers: ashoat, adrian

Differential Revision: https://phab.comm.dev/D4621