[Cases] Fix useGetCaseConnectors flaky test (elastic#184430)

## Summary

Following @JiaweiWu advice I replaced `waitForNextUpdate` with
`waitFor`.

Fixes: elastic#174356

### 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)

x-pack/plugins/cases/public/containers/use_get_case_connectors.test.tsx

@@ -15,8 +15,7 @@ import { useGetCaseConnectors } from './use_get_case_connectors';
 jest.mock('./api');
 jest.mock('../common/lib/kibana');
 
-// FLAKY: https://github.com/elastic/kibana/issues/174356
-describe.skip('useGetCaseConnectors', () => {
+describe('useGetCaseConnectors', () => {
   const caseId = 'test-id';
   const abortCtrl = new AbortController();
   const addSuccess = jest.fn();
@@ -31,11 +30,13 @@ describe.skip('useGetCaseConnectors', () => {
 
   it('calls getCaseConnectors with correct arguments', async () => {
     const spyOnGetCases = jest.spyOn(api, 'getCaseConnectors');
-    const { waitForNextUpdate } = renderHook(() => useGetCaseConnectors(caseId), {
+    const { waitFor } = renderHook(() => useGetCaseConnectors(caseId), {
       wrapper: appMockRender.AppWrapper,
     });
 
-    await waitForNextUpdate();
+    await waitFor(() => {
+      expect(spyOnGetCases).toHaveBeenCalled();
+    });
 
     expect(spyOnGetCases).toBeCalledWith('test-id', abortCtrl.signal);
   });
@@ -49,11 +50,12 @@ describe.skip('useGetCaseConnectors', () => {
     const addError = jest.fn();
     (useToasts as jest.Mock).mockReturnValue({ addSuccess, addError });
 
-    const { waitForNextUpdate } = renderHook(() => useGetCaseConnectors(caseId), {
+    const { waitFor } = renderHook(() => useGetCaseConnectors(caseId), {
       wrapper: appMockRender.AppWrapper,
     });
 
-    await waitForNextUpdate();
-    expect(addError).toHaveBeenCalled();
+    await waitFor(() => {
+      expect(addError).toHaveBeenCalled();
+    });
   });
 });