[libcxx] [test] Avoid mixing assignment and returning.

[STL_MSFT]

Summary

Avoid mixing assignment and returning. "return locked = true;" looks really suspicious, even though it happens to be correct here. Spend an extra statement to make the code clearer, and compilers happier.

Fixes MSVC warning C6282 "Incorrect operator: assignment of constant in Boolean context. Consider using '==' instead."

[EricWF]

I think your compiler is silly, but I'm happy with the change anyway.

I like that syntax gosh darn it!

[EricWF]

r273349.

test/std/thread/thread.mutex/thread.lock/thread.lock.guard/variadic_adopt_lock.pass.cpp

 #include <mutex>
 #include <cassert>
 
 struct TestMutex {
     bool locked = false;
     TestMutex() = default;
 
     void lock() { assert(!locked); locked = true; }
-    bool try_lock() { if (locked) return false; return locked = true; }
+    bool try_lock() { if (locked) return false; locked = true; return true; }
     void unlock() { assert(locked); locked = false; }
 
     TestMutex(TestMutex const&) = delete;
     TestMutex& operator=(TestMutex const&) = delete;
 };
 
 int main()
 {

test/std/thread/thread.mutex/thread.lock/thread.lock.guard/variadic_mutex.pass.cpp

 #include "test_macros.h"
 
 struct TestMutex {
     bool locked = false;
     TestMutex() = default;
     ~TestMutex() { assert(!locked); }
 
     void lock() { assert(!locked); locked = true; }
-    bool try_lock() { if (locked) return false; return locked = true; }
+    bool try_lock() { if (locked) return false; locked = true; return true; }
     void unlock() { assert(locked); locked = false; }
 
     TestMutex(TestMutex const&) = delete;
     TestMutex& operator=(TestMutex const&) = delete;
 };
 
 #if !defined(TEST_HAS_NO_EXCEPTIONS)
 struct TestMutexThrows {