refactor: move utils tests

Author: francoischalifour

packages/autocomplete-core/src/__tests__/utils.test.ts renamed to packages/autocomplete-core/src/utils/__tests__/flatten.test.ts

@@ -1,4 +1,4 @@
-import { flatten } from '../utils';
+import { flatten } from '../flatten';
 
 describe('flatten', () => {
   it('does not split strings', () => {