Add Fill in the Blank test files

[aerenchyma]

Primary question: Does all that sphinx stuff and all that javascript to build the book need to be committed for each test? @bnmnetp

There are four tests here as a start for fill in the blank directive.

I haven't tested e.g. complex regexp, trusting that if a simple one works a complex one works, but let me know if there's a reason to be concerned about that functionality or any more on first thought (cc everyone interested).