Testrunner: show response info

**Changelog**:
* Show response headers and response body.
* Still run tests synchronously (#211) but do not freeze UI.
* Highlight code fragments using [highlightjs](https://highlightjs.org/).
* Minor template fixes: right align buttons, set width of test names; so the layout is not broken when clicking tabs in error message.

![Testrunner](https://i.imgur.com/ZYUctjc.png)