ZTS: Waiting for zvols to be available

This is a follow up patch for PR #12515 which addresses some
additional ZTS tests which are unreliable are should explicitly
wait for the required zvols to be available.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: @Theo13111
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12553