Test Dependencies are now excessive

[kentfredric]

Prior to merging #9, you were using a combination of testing plugins that made it so "authorship" tests were required for end users to run, and subsequently, the choice of plugins you had caused installation to failure if you didn't satisfy those deps.

Subsequently, in 0.17, you changed that, and authortests now ship in xt, and their dependencies are no longer necessary.

However, you still have this set of "user must install these" test dependencies, which is now just silly:

https://github.com/nigelm/html-scrubber/blob/master/dist.ini#L12-L23

Most of these are now only development deps.