test: fix some type errors in test files (#7486)

jest/deps.d.ts

@@ -0,0 +1,17 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+// modules only used in tests
+
+declare module 'to-vfile';
+
+declare module 'remark-mdx';
+
+declare module '@testing-utils/git' {
+  const createTempRepo: typeof import('./utils/git').createTempRepo;
+  export {createTempRepo};
+}