Add test that detects problems in the SMB signing code when run through cli_write...

Add test that detects problems in the SMB signing code when run through cli_write() (which doesn't use the new async methods).

Unfortunately, and I think due to the socket wrapper code, this doesn't
detect the failure on the build farm, but running the RW-SIGNING test
separately against a Samba or Windows server using signing does.

Jeremy.