[Cases] Fix columnsPopover render flaky test (elastic#175778)

## Summary

Fixes elastic#174682


### Checklist

- [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

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

@@ -8,6 +8,7 @@
 import React from 'react';
 import { screen, waitFor } from '@testing-library/react';
 import userEvent from '@testing-library/user-event';
+import { waitForEuiPopoverOpen } from '@elastic/eui/lib/test/rtl';
 
 import type { AppMockRenderer } from '../../common/mock';
 import { createAppMockRenderer } from '../../common/mock';
@@ -34,6 +35,8 @@ describe('ColumnsPopover', () => {
 
     userEvent.click(await screen.findByTestId('column-selection-popover-button'));
 
+    await waitForEuiPopoverOpen();
+
     expect(await screen.findByTestId('column-selection-popover')).toBeInTheDocument();
 
     selectedColumns.forEach(({ field, name, isChecked }) => {
@@ -132,6 +135,7 @@ describe('ColumnsPopover', () => {
     );
 
     userEvent.click(await screen.findByTestId('column-selection-popover-button'));
+    await waitForEuiPopoverOpen();
     userEvent.paste(await screen.findByTestId('column-selection-popover-search'), 'Title');
 
     expect(await screen.findByTestId('column-selection-switch-title')).toBeInTheDocument();
@@ -150,9 +154,12 @@ describe('ColumnsPopover', () => {
     );
 
     userEvent.click(await screen.findByTestId('column-selection-popover-button'));
+    await waitForEuiPopoverOpen();
     userEvent.paste(await screen.findByTestId('column-selection-popover-search'), 'Category');
 
-    expect(onSelectedColumnsChange).not.toHaveBeenCalled();
+    await waitFor(() => {
+      expect(onSelectedColumnsChange).not.toHaveBeenCalled();
+    });
   });
 
   it('searching for text hides the drag and drop icons', async () => {
@@ -177,6 +184,9 @@ describe('ColumnsPopover', () => {
     );
 
     userEvent.click(await screen.findByTestId('column-selection-popover-button'));
+
+    await waitForEuiPopoverOpen();
+
     userEvent.paste(await screen.findByTestId('column-selection-popover-search'), 'Foobar');
 
     expect(await screen.findByTestId('column-selection-popover-show-all-button')).toBeDisabled();