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.