Add unit test for QString::replace with out-of-bounds position

QString::replace(pos, len, *unicode, size) can handle positions
which are outside of the this-string. In that case, it is a no-op.
Coverage analysis revealed we do not have a unit test for this.
This patch adds one.

Change-Id: Id4a407e860fff0d5c7c0a200c379e5e3961c86d2
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

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

@@ -2748,6 +2748,16 @@ void tst_QString::replace_uint_uint_extra()
         s.replace( 0, 3, smallReplacement );
         QCOMPARE( s, smallReplacement );
     }
+
+    {
+        QString s;
+        s.insert(0, QLatin1String("BBB"));
+
+        auto smallReplacement = QString("C");
+
+        s.replace( 5, 3, smallReplacement );
+        QCOMPARE( s, QLatin1String("BBB") );
+    }
 }
 
 void tst_QString::replace_extra()