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/cloud_start_trial/cloud_start_trial_btn.test.tsx

@@ -58,7 +58,7 @@ describe('components/cloud_start_trial_btn/cloud_start_trial_btn', () => {
             modals: {
                 modalState: {
                     learn_more_trial_modal: {
-                        open: 'true',
+                        open: true,
                     },
                 },
             },

webapp/channels/src/components/cloud_start_trial/request_business_email_modal.test.tsx

@@ -40,7 +40,7 @@ describe('components/request_business_email_modal/request_business_email_modal',
             modals: {
                 modalState: {
                     request_business_email_modal: {
-                        open: 'true',
+                        open: true,
                     },
                 },
             },