[libc][NFC] Convert threads unittests in to integration tests.

This is mostly a mechanical change. In a future pass, all tests from
pthread which create threads will also be converted to integration tests.

Some of thread related features are tightly coupled with the loader. So,
they can only be tested with the in-house loader. Hence, going forward, all
tests which create threads will have to be integration tests.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D128381