[libc++][test] Avoid -Wunused-variable warnings in mutex tests (#94907)

After enhancing MSVC's STL to statically initialize our condition_variable,
Clang began noticing that some mutexes in the test suite were unused.

Author: StephanTLavavej

libcxx/test/std/thread/thread.mutex/thread.mutex.requirements/thread.sharedtimedmutex.requirements/thread.sharedtimedmutex.class/default.pass.cpp

@@ -19,9 +19,9 @@
 
 #include "test_macros.h"
 
-int main(int, char**)
-{
-    std::shared_timed_mutex m;
+int main(int, char**) {
+  std::shared_timed_mutex m;
+  (void)m;
 
   return 0;
 }

libcxx/test/std/thread/thread.mutex/thread.mutex.requirements/thread.timedmutex.requirements/thread.timedmutex.class/default.pass.cpp

@@ -18,9 +18,9 @@
 
 #include "test_macros.h"
 
-int main(int, char**)
-{
-    std::timed_mutex m;
+int main(int, char**) {
+  std::timed_mutex m;
+  (void)m;
 
   return 0;
 }

libcxx/test/std/thread/thread.mutex/thread.mutex.requirements/thread.timedmutex.requirements/thread.timedmutex.recursive/default.pass.cpp

@@ -18,9 +18,9 @@
 
 #include "test_macros.h"
 
-int main(int, char**)
-{
-    std::recursive_timed_mutex m;
+int main(int, char**) {
+  std::recursive_timed_mutex m;
+  (void)m;
 
   return 0;
 }