[lib] Replace threadTypes.LOCAL in tests

Summary:
I'm preparing to finally migrate threadTypes.LOCAL to be used for thick threads.

Most of the current uses in the codebase (based on git grep) are in tests. I don't think it matters for these tests, so I'm just going to replace them with threadTypes.COMMUNITY_SECRET_SUBTHREAD (currently they're both 4) so I don't have to worry about them in later diffs.

Depends on D12480

Test Plan: Flow and running unit tests

Reviewers: kamil, marcin

Reviewed By: kamil

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12481