[libc++][test] Fix race condition in condition_variable_any tests (llvm#84788)

Some tests in `condition_variable_any` use two `shared_lock` to guard,
which does not work.
The fix is to make the writer to use `unique_lock`

Author: huixie90

libcxx/test/std/thread/thread.condition/thread.condition.condvarany/wait_for_token_pred.pass.cpp

@@ -119,7 +119,7 @@ void test() {
     bool flag   = false;
     auto thread = support::make_test_thread([&]() {
       std::this_thread::sleep_for(2ms);
-      Lock lock2{mutex};
+      std::unique_lock<Mutex> lock2{mutex};
       flag = true;
       cv.notify_all();
     });

libcxx/test/std/thread/thread.condition/thread.condition.condvarany/wait_token_pred.pass.cpp

@@ -63,7 +63,7 @@ void test() {
     bool flag   = false;
     auto thread = support::make_test_thread([&]() {
       std::this_thread::sleep_for(std::chrono::milliseconds(2));
-      Lock lock2{mutex};
+      std::unique_lock<Mutex> lock2{mutex};
       flag = true;
       cv.notify_all();
     });

libcxx/test/std/thread/thread.condition/thread.condition.condvarany/wait_until_token_pred.pass.cpp

@@ -119,7 +119,7 @@ void test() {
     bool flag   = false;
     auto thread = support::make_test_thread([&]() {
       std::this_thread::sleep_for(std::chrono::milliseconds(2));
-      Lock lock2{mutex};
+      std::unique_lock<Mutex> lock2{mutex};
       flag = true;
       cv.notify_all();
     });