Lacks docs/checks on what programs Subplot needs

I just updated to latest main, 6ee2450e, and ran ./check. It failed while running Subplot scenarios. Thanks to test.log, I eventually figured out that I need to install summain. A quick apt install did that, and after that, ./check succeeded.

I think that Obnam should, at the very least, document which helpers it needs. Better yet, the Subplot test program should check for those programs, and print an error message if some are missing.