Improve types of testing utilities and fix some types in unit test code (#26122)

* Improve type of DeepPartial and make mergeObjects handle Sets/Maps properly

* Fix linting

* Remove undefined fields from menu_cloud_trial.test

* Prevent importing the wrong version of DeepPartial

webapp/channels/src/components/external_link/external_link.test.tsx

@@ -4,7 +4,8 @@
 import {mount} from 'enzyme';
 import React from 'react';
 import {Provider} from 'react-redux';
-import type {DeepPartial} from 'redux';
+
+import type {DeepPartial} from '@mattermost/types/utilities';
 
 import {renderWithContext, screen} from 'tests/react_testing_utils';
 import mockStore from 'tests/test_store';