mk: add PLAIN_TESTS_PORCH support

porch(1), available via sysutils/porch, is an expect(1)-like program
that uses lua instead of tcl for scripting. Imminent use will be to
execute tests of tty canonicalization to ensure we don't regress some of
the corner cases we've fixed, but tests for other interactive programs
are being considered as well.

PLAIN_TESTS_PORCH is being introduced primarily to get the metadata
right without writing it out for every single test; required_programs
ensures that we skip the tests if the port is not installed.

Reviewed by: 0mp (previous version), ngie
Differential Revision: https://reviews.freebsd.org/D46805