[lldb] Stabilize threaded windows lldb-server tests

The tests enabled in 9e699595 are not passing reliably -- sometimes they
report seeing fewer threads than expected.

Based on my (limited) research, this is not a lldb bug, but simply a
consequence of the operating system reporting their presence
asynchronously -- they're reported when they are scheduled to run (or
something similar), and not at the time of the CreateThread call.

To fix this, I add some additional synchronization to the test inferior,
which makes sure that the created thread is alive before continuing to
do other things.

lldb/test/API/tools/lldb-server/main.cpp

@@ -3,6 +3,7 @@
 #include <cstdlib>
 #include <cstring>
 #include <errno.h>
+#include <future>
 #include <inttypes.h>
 #include <memory>
 #include <mutex>
@@ -158,7 +159,8 @@ static void hello() {
   printf("hello, world\n");
 }
 
-static void *thread_func(void *arg) {
+static void *thread_func(std::promise<void> ready) {
+  ready.set_value();
   static std::atomic<int> s_thread_index(1);
   const int this_thread_index = s_thread_index++;
   if (g_print_thread_ids) {
@@ -328,7 +330,10 @@ int main(int argc, char **argv) {
         _exit(0);
 #endif
     } else if (consume_front(arg, "thread:new")) {
-        threads.push_back(std::thread(thread_func, nullptr));
+      std::promise<void> promise;
+      std::future<void> ready = promise.get_future();
+      threads.push_back(std::thread(thread_func, std::move(promise)));
+      ready.wait();
     } else if (consume_front(arg, "thread:print-ids")) {
       // Turn on thread id announcing.
       g_print_thread_ids = true;