[EuiDataGrid] Add remaining unit tests for focus logic (elastic#5698)

* Add unit tests for useHeaderFocusWorkaround

* Add unit tests for preventTabbing

+ remove unnecessary role=gridcell check - after inspecting our DOM, that role attr is set on the topmost cell wrapper but `data-datagrid-cellcontent` is set on a div below that, so there's no way the parent cell should be inside the cell content

+ swap dataset check for hasAttribute - for some reason Jest/JSDOM was failing to register dataset, but hasAttribute works and should be equivalent

* Add unit tests for createKeyDownHandler

* Add unit tests for notifyCellOfFocusState

* Add unit tests for onFocus

+ change unmount_enzyme helper to run after each file instead of each test, to allow onFocusUpdate tests to retain shared state between each it() block

* Fix failing tests caused by unmount_enzyme change

* Fix comment typo

scripts/jest/setup/unmount_enzyme.js

@@ -11,7 +11,7 @@ jest.mock('enzyme', () => {
   };
 });
 
-afterEach(() => {
+afterAll(() => {
   while (mockMountedComponents.length) {
     const component = mockMountedComponents.pop();
     if (component.length === 1) {

src/components/datagrid/body/header/data_grid_header_cell_wrapper.test.tsx

@@ -110,6 +110,9 @@ describe('EuiDataGridHeaderCellWrapper', () => {
       expect(headerCell.getAttribute('tabIndex')).toEqual('-1');
     };
 
+    // Reset focus between tests
+    beforeEach(() => (document.activeElement as HTMLElement)?.blur());
+
     describe('isFocused context', () => {
       describe('when true', () => {
         it('focuses the interactive cell children when present', () => {