These tests use NM threads but NT is not freed for MN thread, causing it
to be reported as memory leaks in LSAN. For example:

    #1 0x62ee7bc67e99 in calloc1 gc/default/default.c:1495:12
    #2 0x62ee7bc7ba00 in rb_gc_impl_calloc gc/default/default.c:8216:5
    #3 0x62ee7bc631d1 in ruby_xcalloc_body gc.c:5221:12
    #4 0x62ee7bc5cdbc in ruby_xcalloc gc.c:5215:34
    #5 0x62ee7bdea4c6 in native_thread_alloc thread_pthread.c:2187:35
    #6 0x62ee7bdec31b in native_thread_check_and_create_shared thread_pthread_mn.c:429:39
    #7 0x62ee7bdea484 in native_thread_create_shared thread_pthread_mn.c:531:12
    #8 0x62ee7bdea1da in native_thread_create thread_pthread.c:2403:16
    #9 0x62ee7bdde2eb in thread_create_core thread.c:884:11
    #10 0x62ee7bde4466 in thread_initialize thread.c:992:16

Introduce `rb_thread_lock_native_thread()` to allocate dedicated
native thread to the current Ruby thread for M:N threads.
This C API is similar to Go's `runtime.LockOSThread()`.

Accepted at https://github.com/ruby/dev-meeting-log/blob/master/2023/DevMeeting-2023-08-24.md
(and missed to implement on Ruby 3.3.0)