ZTS: alloc_class.ksh must wait for the process to exit

The alloc_class_* tests may fail on Linux with an EBUSY error if
`zfs destroy` is run before the `dd` process has had a chance to
terminate.  Wait on the pid after the `kill -9` to make sure.

When testing I didn't observe any failures for the alloc_class
tests.  Remove them from the exceptions list, the CI was used to
verify the tests pass on all platforms.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12873

tests/zfs-tests/tests/functional/alloc_class/alloc_class.kshlib

@@ -62,6 +62,7 @@ function display_status
 	((ret |= $?))
 
 	kill -9 $pid
+	wait $pid 2> /dev/null
 
 	return $ret
 }