Add test for openFiles()

The basic mainWindow methods openFiles(), openDirectories(), isUrlOpen(), and isItemVisibleInAnyView() have some non-obvious behaviours that shouldn't regress. We never want to open directories that are already open. We only want to open new tabs when the currently open ones don't have the file that should be highlighted in view already. This should work even if view modes that allow folder expansion are in play. It should also work when the view area is split.