allows for __tests__ & __test__ directories (#4438)

* fixes #4435 allows for __tests__ directories

* tests hard-coded __tests__

* runs changeset

* Better release notes

* allows `__test__` in addition to `__tests__`

.changeset/modern-cameras-fetch.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+allow files and directories named `__tests__` and `__test__` in the routes directory

packages/kit/src/core/sync/create_manifest_data/index.js

@@ -25,7 +25,7 @@ import { parse_route_id } from '../../../utils/routing.js';
  * }} Item
  */
 
-const specials = new Set(['__layout', '__layout.reset', '__error']);
+const specials = new Set(['__layout', '__layout.reset', '__error', '__tests__', '__test__']);
 
 /**
  * @param {{

packages/kit/src/core/sync/create_manifest_data/index.spec.js

@@ -457,6 +457,12 @@ test('errors on encountering an illegal __file', () => {
 	);
 });
 
+test('allows for __tests__ directories', () => {
+	const { routes } = create('samples/legal-dunder');
+
+	assert.equal(routes, []);
+});
+
 test('creates param matchers', () => {
 	const { matchers } = create('samples/basic'); // directory doesn't matter for the test