Step 2 - Setup `useFakeDate` by default in Jest

• Step 1 - Add fake_date test helper
• Step 2 - Setup useFakeDate by default in Jest
• Step 3 - Update default fake date to match rspec fixtures
• Step 3 - Add helper for useFixturesFakeDate

What does this MR do?

This MR sets up useFakeDate by default in our Jest environment

doc/development/testing_guide/frontend_testing.md

@@ -651,15 +651,27 @@ Non-determinism is the breeding ground for flaky and brittle specs. Such specs e
 
 ### Faking `Date` for determinism
 
-Consider using `useFakeDate` to ensure a consistent value is returned with every `new Date()` or `Date.now()`.
+`Date` is faked by default in our Jest environment. This means every call to `Date()` or `Date.now()` returns a fixed deterministic value.
+
+If you really need to change the default fake date, you can call `useFakeDate` within any `describe` block:
 
 ```javascript
 import { useFakeDate } from 'helpers/fake_date';
 
-describe('cool/component', () => {
-  useFakeDate();
+// NOTE: `useFakeDate` cannot be called during test execution (i.e. inside `it`, `beforeEach`, `beforeAll`, etc.).
+describe("on Ada Lovelace's Birthday", () => {
+  useFakeDate(1815, 11, 10)
+});
+```
 
-  // ...
+If you really need to use the real `Date` class, then you can import and call `useRealDate` within a `describe` block:
+
+```javascript
+import { useRealDate } from 'helpers/fake_date';
+
+// NOTE: `useRealDate` cannot be called during test execution (i.e. inside `it`, `beforeEach`, `beforeAll`, etc.). 
+describe('with real date', () => {
+  useRealDate();
 });
 ```