prettier 2 needs a .js extension

Author: Janther

tests/integration/test-app.js

@@ -1,4 +1,4 @@
-import prettier from 'prettier/standalone';
+import prettier from 'prettier/standalone.js';
 import solidityPlugin from '../../dist/standalone.cjs';
 
 export default async function format(code) {

tests/unit/prettier-version/proxyquire-plugin.cjs

@@ -1,5 +1,5 @@
 const proxyquire = require('proxyquire');
-const prettier = require('prettier/standalone');
+const prettier = require('prettier/standalone.js');
 
 const proxyquirePlugin = () => {
   const prettierMock = { ...prettier, version: '2.2.1', '@global': true };