ZTS: Improve block_device_wait on FreeBSD

FreeBSD doesn't have an equivalent to udevadm settle, so we have been
resorting to a three second sleep to wait for device changes to take
effect. This is far from ideal.

We are mainly waiting for volmode=geom zvols to appear in /dev, so as
a hack, reading the geom config will have the desired effect of
quiescing the geom state.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10768