Fix some browser test suites

The load() method used in tests used the ```fs.readFileSync``` API which I shimmed in our tests with a synchronous XHR now.

This gets us for example:
```
level2/html      711/718      99%
```
(up from 0/718)

Haven't tried to enable the others yet.
Before, these would just output "undefined undefined" and a really badly formatted stack trace. The error hash didn't seem to contain anything when JSON.stringified. There is a message and method property on the root assertion object though.

I'm going to recheck a bit later if the newly generated stack trace has some worth to it.