tst_QString: fix custom QCOMPARE for strings > 2Gi characters

Same fix as in tst_qbytearray's QCOMPARE() in
cb9715557c.

Pick-to: 6.4 6.2
Change-Id: I2222d9015ae7121a2fbcf5b936b27de20e873064
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

tests/auto/corelib/text/qstring/tst_qstring.cpp

@@ -637,7 +637,7 @@ QString verifyZeroTermination(const QString &str)
     if (!strDataPtr->isMutable())
         return str;
 
-    int strSize = str.size();
+    qsizetype strSize = str.size();
     QChar strTerminator = str.constData()[strSize];
     if (QChar('\0') != strTerminator)
         return QString::fromLatin1(