[Cases] fix description user actions flaky test (elastic#170324)

## Summary

I removed the description user actions tests from
`x-pack/plugins/cases/public/components/case_view/components/case_view_activity.test.tsx`

There were a few things being tested:

| Old Test  | Where is it covered |
| ------------- | ------------- |
| should render the description correctly | added description
integration on case view activity page `should render the activity
content and main components` , text of the description is already
verified in
`x-pack/plugins/cases/public/components/user_actions/description.test.tsx`
|
| renders edit description user action correctly | Already testing
description create and update user actions in `renders user actions
correctly` in file
`x-pack/plugins/cases/public/components/user_actions/user_actions_list.test.tsx`
|

Fixes elastic#151981

### Checklist

Delete any items that are not applicable to this PR.

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

### For maintainers

- [x] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

(cherry picked from commit 1d337e3)

x-pack/plugins/cases/public/components/case_view/components/case_view_activity.test.tsx

@@ -179,6 +179,7 @@ describe('Case View Page activity tab', () => {
 
     expect(await screen.findByTestId('case-view-activity')).toBeInTheDocument();
     expect(await screen.findAllByTestId('user-actions-list')).toHaveLength(2);
+    expect(await screen.findByTestId('description')).toBeInTheDocument();
     expect(await screen.findByTestId('case-tags')).toBeInTheDocument();
     expect(await screen.findByTestId('cases-categories')).toBeInTheDocument();
     expect(await screen.findByTestId('connector-edit-header')).toBeInTheDocument();
@@ -417,38 +418,7 @@ describe('Case View Page activity tab', () => {
       });
     });
 
-    // FLAKY: https://github.com/elastic/kibana/issues/151981
-    describe.skip('User actions', () => {
-      it('renders the description correctly', async () => {
-        appMockRender = createAppMockRenderer();
-        appMockRender.render(<CaseViewActivity {...caseProps} />);
-
-        const description = within(await screen.findByTestId('description'));
-
-        expect(await description.findByText(caseData.description)).toBeInTheDocument();
-      });
-
-      it('renders edit description user action correctly', async () => {
-        useFindCaseUserActionsMock.mockReturnValue({
-          ...defaultUseFindCaseUserActions,
-          data: {
-            userActions: [
-              getUserAction('description', 'create'),
-              getUserAction('description', 'update'),
-            ],
-          },
-        });
-
-        appMockRender = createAppMockRenderer();
-        appMockRender.render(<CaseViewActivity {...caseProps} />);
-
-        const userActions = within((await screen.findAllByTestId('user-actions-list'))[1]);
-
-        expect(
-          userActions.getByTestId('description-update-action-description-update')
-        ).toBeInTheDocument();
-      });
-
+    describe('User actions', () => {
       it('renders the unassigned users correctly', async () => {
         useFindCaseUserActionsMock.mockReturnValue({
           ...defaultUseFindCaseUserActions,