tests/vm/freebsd: Update to FreeBSD 13.2

tests/vm/freebsd: Update to FreeBSD 13.2

According to QEMU's support policy, we stop supporting the previous
major release two years after the the new major release has been
published. So we can stop testing FreeBSD 12 now and should switch
our FreeBSD VM to version 13 instead.

Some changes are needed for this update: The downloadable .ISO images
do not use the serial port as console by default anymore, so they
are not usable in the same way as with FreeBSD 12. Fortunately, the
FreeBSD project now also offers some pre-installed CI images that
have the serial console enabled, so we can use those now, with the
benefit that we can skip almost all parts of the previous installation
process.

Message-Id: <20230419144553.719749-1-thuth@redhat.com>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>