tst_qmlformat: fix an uninitialized variable usage

GCC rightfully complains that

    /home/qt/work/qt/qtdeclarative/tests/auto/qml/qmlformat/tst_qmlformat.cpp:263:21: warning: ‘writtenOut’ may be used uninitialized in this function [-Wmaybe-uninitialized]
      263 |         *didSucceed = writtenOut;
          |         ~~~~~~~~~~~~^~~~~~~~~~~~

because if we don't enter the if, we never initialize writtenOut.
Initialize it.

Pick-to: 6.9 6.8
Change-Id: I6d3bb8038e524773a11b3333bf65654593f01da9
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

tests/auto/qml/qmlformat/tst_qmlformat.cpp

@@ -242,7 +242,7 @@ QString TestQmlformat::formatInMemory(const QString &fileToFormat, bool *didSucc
     env->loadPendingDependencies();
     MutableDomItem myFile = tFile.field(Fields::currentItem);
 
-    bool writtenOut;
+    bool writtenOut = false;
     QString resultStr;
     if (myFile.field(Fields::isValid).value().toBool()) {
         WriteOutChecks checks = extraChecks;