Test: cts-schemas: Add test for sorting nvsets

All nvset elements (cluster_property_set, instance_attributes,
meta_attributes, utilization) should be sorted by score and placed below
their non-nvset siblings. This is not necessary for conformance to the
4.0 schema, but it will simplify future transformations without changing
behavior.

The result is not yet as desired.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>