ZTS: Move dumpdev tests to sunos.run

ZTS: Move dumpdev tests to sunos.run

Neither FreeBSD nor Linux support dumping to zvols.

DilOS still uses these tests, so the files are kept and the tests have
been relocated to sunos.run.

An `is_illumos` function was added to libtest.shlib to eliminate some
awkward platform checks.

A few functions that are not expected to be used outside of illumos
have been sanitized of extraneous FreeBSD adaptations.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9794