feature(test): Add a `/clear` route that clears browser state.

* Used in the selenium tests instead of running `unsafeEval` or `eval`

fixes #1285 

@zaach, @pdehaan or @vladikoff - wanna have a look?
I'm just quickly/superficially reading through this, but is there a difference between using `self` above but `this` here? Or change this to `self` for consistency? "STFU" is also a valid response.