tests: kern: add some porch(1)-based tty tests

If sysutils/porch is installed, we'll do some basic testing of tty
behavior. The existing tests primarily cover ICANON-related processing
and corner cases that have been fixed somewhat recently, but I
anticipate growing this out a bit in due time.

Reviewed by: ngie
Differential Revision: https://reviews.freebsd.org/D46806