Freeze certain tests

[jxxcarlson]

I'd like to suggest that there be way to mark certaion tests as 'frozen' -- perhaps there could be a set of directores that have this property. In working on the tests for Asciidoctor-LateX, I have to regenerate tests from time to time. But there are certain tests which I want to remain untouched unless I manally change them.

I do have mechanism in place for this but it is not the greatest, and it may be a need that other doctest users have also.