Replacing Karma with Web Test Runner

[koddsson]

Web Test Runner is a pretty good test runner that runs tests in browsers and a good replacement for Karma.

[koddsson]

Looks like Safari has some nonstandard sourceURL property that's showing up in the AssertionError. Looking into this.

[koddsson]

I actually just dropped checking in Safari. These issues also exist on branch 5.x.x but it looks like we've never tested in Safari before. Since WebKit uses a different standard than V8 we'd need to normalise the stack trace somehow but I'd say that it's out of scope for this work.

Reference: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error/stack