[Cases] Unskip severity flaky test (#177597)

## Summary
As a follow-up to #176863 I am unskipping these severity tests to check
if the changes on that PR stabilized them.

Fixes #176336

Author: adcoelho

x-pack/plugins/cases/public/components/all_cases/severity_filter.test.tsx

@@ -14,8 +14,7 @@ import { screen, waitFor } from '@testing-library/react';
 import { waitForEuiPopoverOpen } from '@elastic/eui/lib/test/rtl';
 import { SeverityFilter } from './severity_filter';
 
-// FLAKY: https://github.com/elastic/kibana/issues/176336
-describe.skip('Severity form field', () => {
+describe('Severity form field', () => {
   const onChange = jest.fn();
   let appMockRender: AppMockRenderer;
   const props = {