QSignalSpy: make the mutex a member variable

The static inline was only a workaround for maintaining BC within
minor releases while backporting the fix. Since we don't promise BC for
QtTest between minor releases, we can make the mutex a proper member
variable for Qt 6.8.

Amends c837cd7593.

Change-Id: I0d6353bdd6a11daa4f927139abf9a867d8c9f95f
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/testlib/qsignalspy.h

@@ -215,7 +215,7 @@ private:
 
     QTestEventLoop m_loop;
     bool m_waiting;
-    static inline QMutex m_mutex; // protects m_waiting, args and the QList base class, between appendArgs() and wait()
+    QMutex m_mutex; // protects m_waiting, args and the QList base class, between appendArgs() and wait()
 };
 
 QT_END_NAMESPACE