Fix incorrect cond construction in _minimal_valueset_satisfying_constraint

Fix incorrect cond construction in _minimal_valueset_satisfying_constraint

The function in question introduced in d681f1bb (which builds upon work in
8e40a627) correctly uses _extract_fixed_condition_columns, but then fails to
account for all literals having their leading { '=' => ... } being stripped,
nor does it consider UNRESOLVABLE_CONDITION as a valid return value.

Tests and fixes to get this rolling, thanks go to Lianna Eeftinck for testing
and reporting \o/