I cannot create two btrfs subvolumes "a" and "b" for a existing
btrfs file system mounted at /test.

First I create the subvolume "a". Right away I try to create the
subvolume "b". Now YaST complains that only paths starting with
"a/" are allowed and changed the path "b" to "a/b". Then YaST
complains that the mount point "/test/b" is shadowed. For that
should now be "/test/a/b" AFAIS and I cannot see why it is shadowed.