Fix thread id leaks in subthread_create and threadkey_test

Fix thread id leaks in subthread_create and threadkey_test

* tests/subthread_create.c [GC_PTHREADS] (entry): Call pthread_detach
for the created thread.
* tests/threadkey_test.c (main): Likewise.
* tests/test.c [GC_PTHREADS && CPPCHECK] (main): Remove
UNTESTED(GC_pthread_detach).
* tests/threadkey_test.c (on_thread_exit_inner): New local variable
attr; call pthread_attr_init and pthread_attr_setdetachstate;
pass attr to GC_pthread_create; call pthread_attr_destroy.