tee: add some basic tests

The cases are ordered in such a way that we naturally progress through
the functionality, with the earliest failures perhaps shedding light on
any later failures.

sysutils/porch is used for one test if it's available, just to cleanly
check that SIGINT is being ignored properly.

Reviewed by: des, emaste
Differential Revision: https://reviews.freebsd.org/D48195