added netr_ServerAuthenticate() and test code

added netr_ServerAuthenticate() and test code

I would like the netlogon test suite to eventually do a new domain
join using a fake workstation name, then remove itself afterwards, but
for now I'm assuming we are already joined to the domain when the
testsuite runs. This means you need to use the Samba3 net command to
do a join before running RPC-NETLOGON
(This used to be commit 8c7a9446a0892a4f7722cced5019667f7a9fafdd)