test: pin the playlist test locale so sorting does not depend on LANG

autotests/playlistmodeltest.cpp

@@ -33,6 +33,12 @@ private:
     }
 
 private Q_SLOTS:
+    void initTestCase()
+    {
+        // The C locale's collation is case-sensitive, so the sort order would depend on $LANG.
+        QLocale::setDefault(QLocale(QLocale::English, QLocale::UnitedStates));
+    }
+
     void sortsEpisodesNaturallyAndSkipsOtherFiles()
     {
         QTemporaryDir dir;