ZTS: Add helper for disk device check

Replace test -b and equivalents with is_disk_device, so that -c
is used instead on FreeBSD which has no block cache layer for devices.

Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9795