ztest: update expectation for sparing a special device

Commit c23738c70eb86a7f04f93292caef2ed977047608 modified the expected
behavior of attach to prevent hot spares from being used as special
vdev replacements. We update ztest's expectations accordingly to
prevent it from failing when testing the updated behavior.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #14351