Remove transform file .js restriction for testUtils (#308)

Since we are able to write transformers in TS, the .js extension hard coded in testUtils not allow us to use it to write tests for TS transform modules.
And `require('some-module')` will automatically infer some-module's extension whether it is ended with '.js' or '.ts', it should be safe to remove ',js'.

src/testUtils.js

@@ -69,7 +69,7 @@ function runTest(dirName, transformName, options, testFilePrefix) {
     'utf8'
   );
   // Assumes transform is one level up from __tests__ directory
-  const module = require(path.join(dirName, '..', transformName + '.js'));
+  const module = require(path.join(dirName, '..', transformName));
   runInlineTest(module, options, {
     path: inputPath,
     source