porch(1) tty tests

Hello!

Back in January, I sent some mail[0] about using a tool I wrote, 
orch(1), for some tty testing.  It's since been renamed porch(1), but I 
now have some reviews open for this:

  - https://reviews.freebsd.org/D46805 (mk: add PLAIN_TESTS_PORCH support)
  - https://reviews.freebsd.org/D46806 (tests: kern: add some 
porch(1)-based tty tests)

PLAIN_TESTS_PORCH is useful so that we don't need to screw up or repeat 
the required_programs metadata to make sure the test is skipped if 
porch(1) isn't installed.  The only tests currently using it are the tty 
tests that I wrote around canonicalization behavior, but we have other 
interactive programs in base that could benefit as well.

I've only added #tests for now, but please feel free to add yourself to 
the reviews if you're at all interested in tty testing, interactive 
program testing, or even just interactive program orchestration in general.

Thanks,

Kyle Evans

[0] https://lists.freebsd.org/archives/freebsd-arch/2024-January/000581.html