Fix browser page navigation for Puppeteer 11+ in test/test.js

In Puppeteer 11 we noticed that Firefox doesn't shut down once the tests
are done anymore. I tracked this down to the `page.goto` call, in
`startBrowser`, never resolving anymore. I can only assume that
something changed in Puppeteer, possibly in combination with recent
Firefox Nightly versions, that caused this, but haven't been able to
fully track it down.

However, I did find that the problem is that the `load` event no longer
triggers, so fortunately we can fix the problem by explicitly waiting
for the `domcontentloaded` event instead. In general this change might
even be better since we now wait until the test framework is fully
loaded before we continue. Note that this also still works for the
current Puppeteer version.

I did find two upstream references that appear to track this issue, both
on the Puppeteer side and on the Firefox side, making me further suspect
that the issue is partly on both sides:

- https://github.com/puppeteer/puppeteer/issues/5806
- https://bugzilla.mozilla.org/show_bug.cgi?id=1706353

test/test.js

@@ -956,7 +956,7 @@ async function startBrowser(browserName, startUrl = "") {
   if (startUrl) {
     const pages = await browser.pages();
     const page = pages[0];
-    await page.goto(startUrl, { timeout: 0 });
+    await page.goto(startUrl, { timeout: 0, waitUntil: "domcontentloaded" });
   }
 
   return browser;