[Threading] Ensure Tests Reflect Disabled Threads

D137836 changed what llvm::get_physical_cores returns when threads are
disabled, to bring it inline with the other parts of Threading. It now
returns the value for "unknown" when threading is disabled.

This commit updates the tests (which are failing on some platforms), to
also reflect this change.

Differential Revision: https://reviews.llvm.org/D139015