docs(material/tree): show testing API docs (#33862)

The tree was missing `additionalApiDocs` in the docs site config, so the
`MatTreeHarness` and `MatTreeNodeHarness` API docs were generated but never
rendered on the tree API page. Add the missing entry and a test for it.

Fixes #33832

(cherry picked from commit 8d9f52c)

Author: MeAkib

docs/src/app/shared/documentation-items/documentation-items.ts

@@ -401,6 +401,7 @@ const DOCS: {[key: string]: DocItem[]} = {
       exampleSpecs: {
         prefix: 'tree-',
       },
+      additionalApiDocs: [{name: 'Testing', path: 'material-tree-testing.html'}],
     },
   ],
   [CDK]: [