Do not assume transform files are .js

This makes it possible to write codemods with TypeScript using a `.ts` extension with `ts-jest`.

Author: eventualbuddha

src/test-support.js

@@ -13,7 +13,7 @@ function transformDetails(options) {
   return {
     name: options.name,
     root,
-    transformPath: root + 'index.js',
+    transformPath: root + 'index',
     fixtureDir: root + '__testfixtures__/',
   };
 }