tst_QMutex: use constexpr variables instead of 'enum-trick'

C++20 doesn't like arithmetic with enums anymore. While this hasn't
caused immediate pain, yet, fix it pro-actively for an imminent patch.

As a drive-by, fix the missing space at start of comment.

Pick-to: 6.5 6.4 6.4.3 6.2
Change-Id: Id08bb227c587bc7b900c593a7b6d2655ca32eefd
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

tests/auto/corelib/thread/qmutex/tst_qmutex.cpp

@@ -1038,8 +1038,8 @@ void tst_QMutex::lock_unlock_locked_tryLock()
     }
 }
 
-enum { one_minute = 6 * 1000, //not really one minute, but else it is too long.
-       threadCount = 10 };
+constexpr int one_minute = 6 * 1000; // not really one minute, but else it is too long.
+constexpr int threadCount = 10;
 
 class StressTestThread : public QThread
 {