Add test for jsdom config

test/jsdom arg should set jsdom config.js

@@ -0,0 +1,9 @@
+import test from 'ava';
+import browserEnv from '../dist';
+
+test(t => {
+  const userAgent = 'Custom user agent';
+  t.is(typeof navigator, 'undefined');
+  browserEnv(['navigator'], { userAgent: userAgent });
+  t.is(navigator.userAgent, userAgent);
+});