Bump babel-plugin-tester and fix breaking changes

Created by: andriijas

Bump babel-plugin-tester and adapt to new api

https://github.com/babel-utils/babel-plugin-tester/releases

packages/babel-plugin-named-asset-import/index.test.js

-const pluginTester = require('babel-plugin-tester');
+const pluginTester = require('babel-plugin-tester/pure');
 const namedAssetImport = require('./index');
 
-pluginTester({
+pluginTester.default({
   plugin: namedAssetImport,
   pluginOptions: {
     loaderMap: {

packages/babel-plugin-named-asset-import/package.json

@@ -19,7 +19,7 @@
     "@babel/core": "^7.1.0"
   },
   "devDependencies": {
-    "babel-plugin-tester": "^7.0.1",
+    "babel-plugin-tester": "^8.0.1",
     "jest": "24.9.0"
   },
   "scripts": {