fix(spp_mis_demo_v2): address review comments on demo data

Wrap demo_statistics.xml records in <data noupdate="1"> to prevent
overwriting on module update, consistent with other demo files.

Replace test_statistics_accessible_via_aggregation_service with
test_statistics_have_valid_cel_accessors, which tests the correct
property: that each statistic's variable has a cel_accessor configured.
The original test used an incorrect compute_aggregation signature
(registrant_ids kwarg instead of the required scope positional arg).

Author: jeremi