test(test_delete_sources): parameterized functional test for deletion…

… of one or more sources

This is a complete rewrite of this test, which now (a) distinguishes
between single- and multi-select behavior and (b) checks that the
individual sources selected for deletion are confirmed and deleted, not
only the *number* of sources selected.

This includes a workaround for #2273.  Here we also change
securedrop_client.gui.widgets.BatchActionToolbar.on_action_triggered()
to call QDialog.open(), which is modal with QDialog.setModal(), rather
than QDialog.exec(), which is modal *and* blocking, which blocks pytest
timers as well as the main event loop.

client/securedrop_client/gui/widgets.py

@@ -555,8 +555,9 @@ def on_action_triggered(self) -> None:
             targets = self.controller.get_selected_sources()
             if targets is not None:
                 dialog = DeleteSourceDialog(targets)
+                self._last_dialog = dialog  # FIXME: workaround for #2273
                 dialog.accepted.connect(lambda: self.controller.delete_sources(targets))
-                dialog.exec()
+                dialog.open()
             else:
                 # No selected sources should return an empty set, not None
                 logger.error("Toolbar action triggered without valid data from controller.")