Implement testing for the browser target

[TerrorJack]

Our CI config only tests the compiled code for the node target; linking is tested for browser target, but the output code is not run, therefore regressions in the browser target can be introduced without notice. We should run all tests which don't require node-specific imports on the browser target as well. A good first step would be to implement selenium or puppeteer based logic in inline-js for running code in a headless browser.

[TerrorJack]

Some recent updates about the issue:

• Besides the in-tree todomvc, it would be nice to test plzwrk and future user-written frameworks as well. It would be a nice way to prevent (or at least notify) downstream breakage.
• Microsoft has added playwright to the headless testing framework landscape. It has a puppeteer-like modern API, but works with chromium/firefox/webkit. This should be preferred when we actually starting working on this issue.