tests: figure out correct version of sed to run

tests: figure out correct version of sed to run

In run_in_venv_with_cleanup.sh, sed was changed to gsed to allow the
script to run properly on FreeBSD because the sed script uses an
expression that is specific to the gnu sed. Gnu sed is available to
be invoked as gsed on FreeBSD systems, but there is no executable or
symlink which allows sed to be run by the name gsed on ubuntu 22.04.

Check for the existence of gsed. If it's found, use it. Otherwise, just
use sed.

Type: fix
Fixes: b3c863eae4

Signed-off-by: Matthew Smith <mgsmith@netgate.com>
Change-Id: I487197e486f500711aa3e87ec7ba899a53606b40