tests: Add basic test for non-global LoadParm behaviour

tests: Add basic test for non-global LoadParm behaviour

Add a simple test to show that the new non-global LoadParm behaviour
works, i.e.
- by default all LoadParm objects are linked to the same underlying
  object
- using a non-global LoadParm creates a separate underlying object.
- using a non-global LoadParm with a bad filename fails.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>