ZTS: Add missing quotes

ZTS: Add missing quotes

`default_setup` takes a disk list as the first argument and has
optional additional arguments that control secondary functionality.
A couple of test setups mistakenly call `default_setup $DISKS`.

Add quotes so the second and subsequent disks are correctly included
in the pool as vdevs rather than triggering unwanted behavior from
`default_setup`.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10097