python:samba/tests: don't use the x.alter_context() method in dcerpc/bare.py

python:samba/tests: don't use the x.alter_context() method in dcerpc/bare.py

Establishing a new context on a given connection using alter_context
is supposed to be done by using y = ClientConnection(..., basis_connection=x)

The current x.alter_context() can work as it's not allowed to
change the abstract or transfer syntax of an existing presentation
context.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11804

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>
(cherry picked from commit 84993440aa9dadd89d8739102c3b7771774064fa)