ZTS: Test boundary conditions in alloc_class_012

Issue #9142 describes an error in the checks for device removal that
can prevent removal of special allocation class vdevs in some
situations.

Enhance alloc_class/alloc_class_012_pos to check situations where this
bug occurs.

Update zts-report with knowledge of issue #9142.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10116
Issue #9142