Add very basic integration tests

tests/cjs/index.js

@@ -0,0 +1,2 @@
+require('../../lib/index.umd');
+console.log('import successful!');

tests/cjs/package.json

@@ -0,0 +1,4 @@
+{
+  "main": "index.js",
+  "type": "commonjs"
+}