s4/test-libnet: Helper func to initialize libnet_context

s4/test-libnet: Helper func to initialize libnet_context

and create rpc connections to SAMR and LSA pipes.
This function should be used in cases where we need
'deterministic' connection to the DC we are testing against.

The problem is that most of the test follow the pattern:
1. Connect to server supplied on command line
2. Create user/group through connection in 1.
3. Use 'fresh' libnet_context to query/alter the object in
   step 2.

In singe-DC environment this work well. But in multiple-DC
environment libnet may connect to another DC (step 3), not
the one we use in step 1 and 2. This leads to false error -
object created in step 2 is not found on DC we are querying
in step 3.