ZTS: Add known exceptions

The receive-o-x_props_override test case reliably fails on the
FreeBSD main builders (but not on Linux), until the root cause is
understood add this test to the FreeBSD exception list.

On Linux the alloc_class_012_pos test case may occasionally fail.
This is a known false positive which has also been added to the
Linux exception list until the test can be made entirely reliable.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12272