Switch to official build of htmltest

[theunrepentantgeek]

Describe the current behavior

Once #3022 merges, we'll be using a private fork of htmltest that includes support for sites that publish to subfolders.

Describe the improvement

Once PR#215 merges and is released, we should switch back to using the official release.

[matthchr]

Still interested in doing thus but unfortunately the maintainer of htmltest hasn't merged the fixes in a year+

[theunrepentantgeek]

As above.