screenmappertest: Add tests to make sure activity ID works as expected

1. `tst_readScreenActivityMapping()`: Test reading screen mappings from a
   QStringList.
2. `tst_saveScreenActivityMappingFromOldConfig()`: Test reading screen
   mappings from an old config before Plasma 5.25.
3. `tst_readScreenActivityMapping()`: Test reading screen mappings from a
   QStringList.
4. `tst_readAndSaveItemsOnActivitiesOnDisabledScreens()`: Test reading
   and saving disabled screen mappings.
5. `tst_readAndSaveItemsOnActivitiesOnDisabledScreensFromOldConfig()`: Test
   reading and saving disabled screen mappings from an old config before
   Plasma 5.25.