Report version of nmc being tested
Factor out tests to separate files

This may actually have been spread across too many files – we might want
block.at instead of codeblock.at, paragraph.at, … – but let’s start here
and see where we wind up once we clean up the tests themselves.
Fix figcaption output
Add helper macros for tests
Change [[…]] to […] in tests

As far as I can tell, there’s no real benefit of the added quoting for
what we’re doing.
Remove explicit newline from transform input
Move indent-testing tests to indent.at
Move block tests to bol.at
Update indent tests
Further distill section tests
Clean up title tests
Force three spaces after item tags

This makes items line up properly.
Update block tests
Update indent tests
Update code block tests
Update footnote tests
Update XML tests
Update code inline tests
Update emphasis tests
Update inlines tests