Mike Cowlishaw has confirmed that the tests scbx164, scbx165 (in version 
2.59 of the tests) are implementation-specific, so test_decimal is doing 
the right thing in skipping them.  So I think this issue can be closed.

I don't think it's worth removing the restriction on the 2nd scaleb 
argument:  while I still think it's arbitrary and unnecessary, keeping it 
allows us to say that we're in full compliance with the specification.