Add black-box tests for bin/p6doc
Add black-box tests for bin/p6doc

It would take refactoring p6doc entirely to be able to test routines
meaningfully. But black-box testing ensures that it's doing the right
thing, and hopefully is not too brittle so as to cause spurious failures
when unrelated changes are made.

tbrowder/doc,MasterDuke17/doc,tbrowder/doc,tbrowder/doc,MasterDuke17/doc,MasterDuke17/doc,tbrowder/doc,MasterDuke17/doc