pw_thread: Rename test_threads.h, update docs

test_threads.h provides functions intended for testing pw_thread
backends, not for general unit test use. It is not a facade, so using it
requires a platform-specific dependency.

When users are looking for a way to create threads for unit tests,
they're likely to come across test_threads.h and use it, even though
they should be using the TestThreadContext facade in
pw_thread/test_thread_context.h.

- Rename the test_threads.h header to non_portable_test_thread_options.h
  to clarify that it is not a facade and make it less likely to be used
  in unit tests.
- Rename "test_threads" targets to "non_portable_test_thread_options".
- Update the TestThreadContext documentation. Link to it from
  non_portable_test_thread_options.h.

Change-Id: I5e0c9ffcbe083860d2b31ba3885acbe3fb098d1e
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/155130
Commit-Queue: Wyatt Hepler <hepler@google.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Reviewed-by: Aaron Green <aarongreen@google.com>
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>