Text filter bug fix

I found the main bug that was causing problems with that filter.

It was writing out `buffer.Length` bytes every time, when there is a `count` argument passed in with the max no. of bytes to write.

I'm a little concerned that it always uses UTF-8 - I'm not sure if it's possible for MVC to use something other than UTF8. If it can then it will still cause problems in some scenarios.

This PR also includes some changes to start making the Test Harness more useful / understandable. One question I do have - there are heaps of .txt files in there that don't seem to be picked up by Karbon when checking for visible pages, is that intentional? e.g. all the brand.txt ones and everything under blog.
Here is the culprit