feat: add tests for new .mjs and .cjs file extensions

testdata/expected/file.cjs

@@ -0,0 +1,22 @@
+/**
+ * Copyright 2018 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+function dummy() {
+  console.log('hello world!');
+}
+
+module.exports = dummy
+

testdata/expected/file.mjs

@@ -0,0 +1,22 @@
+/**
+ * Copyright 2018 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+function dummy() {
+  console.log('hello world!');
+}
+
+export default dummy
+

testdata/initial/file.cjs

@@ -0,0 +1,6 @@
+function dummy() {
+  console.log('hello world!');
+}
+
+module.exports = dummy
+

testdata/initial/file.mjs

@@ -0,0 +1,6 @@
+function dummy() {
+  console.log('hello world!');
+}
+
+export default dummy
+