Fix skipped Bulk Actions in Alerts Table tests (#156795)

## Summary

Fixes #154970
Fixes #152176

Run times before:
![Screenshot
12](https://user-images.githubusercontent.com/17549662/236402240-5be77baa-c35b-44f2-bbe1-f49d7cd6accc.png)

Run times after:

![Screenshot](https://user-images.githubusercontent.com/17549662/236402290-0d7377b2-1db3-4c62-8164-ff40358aea63.png)

x-pack/plugins/triggers_actions_ui/public/application/sections/alerts_table/bulk_actions/bulk_actions.test.tsx

@@ -61,7 +61,44 @@ jest.mock('@kbn/kibana-react-plugin/public', () => {
   };
 });
 
+const originalGetComputedStyle = Object.assign({}, window.getComputedStyle);
+
 describe('AlertsTable.BulkActions', () => {
+  beforeAll(() => {
+    // The JSDOM implementation is too slow
+    // Especially for dropdowns that try to position themselves
+    // perf issue - https://github.com/jsdom/jsdom/issues/3234
+    Object.defineProperty(window, 'getComputedStyle', {
+      value: (el: HTMLElement) => {
+        /**
+         * This is based on the jsdom implementation of getComputedStyle
+         * https://github.com/jsdom/jsdom/blob/9dae17bf0ad09042cfccd82e6a9d06d3a615d9f4/lib/jsdom/browser/Window.js#L779-L820
+         *
+         * It is missing global style parsing and will only return styles applied directly to an element.
+         * Will not return styles that are global or from emotion
+         */
+        const declaration = new CSSStyleDeclaration();
+        const { style } = el;
+
+        Array.prototype.forEach.call(style, (property: string) => {
+          declaration.setProperty(
+            property,
+            style.getPropertyValue(property),
+            style.getPropertyPriority(property)
+          );
+        });
+
+        return declaration;
+      },
+      configurable: true,
+      writable: true,
+    });
+  });
+
+  afterAll(() => {
+    Object.defineProperty(window, 'getComputedStyle', originalGetComputedStyle);
+  });
+
   const alerts = [
     {
       [AlertsField.name]: ['one'],
@@ -691,8 +728,7 @@ describe('AlertsTable.BulkActions', () => {
           ).toBeTruthy();
         });
 
-        // FLAKY: https://github.com/elastic/kibana/issues/154970
-        describe.skip('and clear the selection is clicked', () => {
+        describe('and clear the selection is clicked', () => {
           it('should turn off the toolbar', async () => {
             const props = {
               ...tablePropsWithBulkActions,
@@ -731,8 +767,7 @@ describe('AlertsTable.BulkActions', () => {
           });
         });
 
-        // FLAKY: https://github.com/elastic/kibana/issues/152176
-        describe.skip('and executing a bulk action', () => {
+        describe('and executing a bulk action', () => {
           it('should return the are all selected flag set to true', async () => {
             const mockedFn = jest.fn();
             const props = {
@@ -781,6 +816,10 @@ describe('AlertsTable.BulkActions', () => {
                     field: 'kibana.alert.rule.uuid',
                     value: ['uuidone'],
                   },
+                  {
+                    field: 'kibana.alert.case_ids',
+                    value: [],
+                  },
                 ],
                 ecs: {
                   _id: 'alert0',
@@ -799,6 +838,10 @@ describe('AlertsTable.BulkActions', () => {
                     field: 'kibana.alert.rule.uuid',
                     value: ['uuidtwo'],
                   },
+                  {
+                    field: 'kibana.alert.case_ids',
+                    value: [],
+                  },
                 ],
                 ecs: {
                   _id: 'alert1',