ZTS: add snapshot/snapshot_002_pos exception

Add snapshot_002_pos to the known list of occasional failures
for FreeBSD until it can be made entirely reliable.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #14831
Closes #14832