[libc++] Introduce an indirection to create threads in the test suite

We create threads using std::thread in various places in the test suite.
However, the usual std::thread constructor may not work on all platforms,
e.g. on platforms where passing a stack size is required to create a thread.

This commit introduces a simple indirection that makes it easier to tweak
how threads are created inside the test suite on various platforms. Note
that tests that are purposefully calling std::thread's constructor directly
(e.g. because that is what they're testing) were not modified.

libcxx/test/std/thread/thread.threads/thread.thread.class/thread.thread.constr/move.pass.cpp

@@ -19,6 +19,7 @@
 #include <cstdlib>
 #include <utility>
 
+#include "make_test_thread.h"
 #include "test_macros.h"
 
 class G
@@ -52,7 +53,7 @@ int main(int, char**)
         assert(G::n_alive == 1);
         assert(!G::op_run);
 
-        std::thread t0(g);
+        std::thread t0 = support::make_test_thread(g);
         std::thread::id id = t0.get_id();
 
         std::thread t1 = std::move(t0);