TestPacketDataStream: use UTF-8 instead of Latin1.

The string test for PacketDataStream uses the string
Bærtur -- but encoded as Latin1.

Keep the string -- it's as good as any -- but use UTF-8 instead,
to make clang happy.

Fixes mumble-voip#2978

src/tests/TestPacketDataStream/TestPacketDataStream.cpp

@@ -93,7 +93,7 @@ void TestPacketDataStream::string_data() {
 	QTest::addColumn<QString>("string");
 	QTest::newRow("Empty") << QString("");
 	QTest::newRow("Null") << QString();
-	QTest::newRow("Bærtur") << QString("Bærtur");
+	QTest::newRow("Bærtur") << QString("Bærtur");
 }
 
 void TestPacketDataStream::string() {