fix(aio): fix PWA testing on Windows (and reduce flaky-ness)

Adding a delay after Lighthouse has run and before killing the Chrome process
avoids `ECONNREFUSED` errors on Windows and will hopefully prevent such errors
occasionally appearing on CI.

Based on info in:
https://github.com/paulirish/pwmetrics/issues/63#issuecomment-282721068

aio/scripts/test-pwa-score.js

@@ -79,6 +79,9 @@ function launchChromeAndRunLighthouse(url, flags, config) {
 
   return launcher.run().
     then(() => lighthouse(url, flags, config)).
+    // Avoid race condition by adding a delay before killing Chrome.
+    // (See also https://github.com/paulirish/pwmetrics/issues/63#issuecomment-282721068.)
+    then(results => new Promise(resolve => setTimeout(() => resolve(results), 1000))).
     then(results => launcher.kill().then(() => results)).
     catch(err => launcher.kill().then(() => { throw err; }, () => { throw err; }));
 }