Lexical: Added some general test guidance

Just to help remember the general layout/methods that we've added to
make testing easier.

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+# Testing Guidance
+
+This is testing guidance specific for this Lexical-based WYSIWYG editor.
+There is a lot of pre-existing test code carried over form the fork of lexical, but since there we've added a range of helpers and altered how testing can be done to make things a bit simpler and aligned with how we run tests.
+
+This document is an attempt to document the new best options for added tests with an aim for standardisation on these approaches going forward.
+
+## Utils Location
+
+Most core test utils can be found in the file at path: resources/js/wysiwyg/lexical/core/__tests__/utils/index.ts
+
+## Test Example
+
+This is an example of a typical test using the common modern utilities to help perform actions or assertions. Comments are for this example only, and are not expected in actual test files.
+
+```ts
+import {
+    createTestContext,
+    dispatchKeydownEventForNode, 
+    expectEditorStateJSONPropToEqual,
+    expectNodeShapeToMatch
+} from "lexical/__tests__/utils";
+import {
+    $getRoot,
+    ParagraphNode,
+    TextNode
+} from "lexical";
+
+describe('A specific service or file or function', () => {
+    test('it does thing', async () => {
+        // Create the editor context and get an editor reference
+        const {editor} = createTestContext();
+
+        // Run an action within the editor.
+        let pNode: ParagraphNode;
+        editor.updateAndCommit(() => {
+            pNode = new ParagraphNode();
+            const text = new TextNode('Hello!');
+            pNode.append(text);
+            $getRoot().append(pNode);
+        });
+
+        // Dispatch key events via the DOM
+        dispatchKeydownEventForNode(pNode!, editor, ' ');
+
+        // Check the shape (and text) of the resulting state
+        expectNodeShapeToMatch(editor, [{type: 'paragraph', children: [
+                {text: 'Hello!'},
+            ]}]);
+
+        // Check specific props in the resulting JSON state
+        expectEditorStateJSONPropToEqual(editor, '0.0.text', 'Hello!');
+    });
+});
+```
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