Test enum and string values in household calculations without axes #4147

PolicyEngineCountry.calculate formats each value of a household calculation without axes by the variable's value_type. Since #4136 moved the SPM country tests onto stand-in simulations, every unit test that calls it uses float or int variables, so no test reaches the Enum branch (result.decode()[entity_index].name) or the str branch (str(result[entity_index])). Only the removed full-model calculations reached them, and codecov/project fell 0.03 points on #4136.

Add a unit test over a stand-in simulation that calculates an enum variable and a string variable for a household without axes, and checks that the result holds the enum member's name and a plain str. The axes path is already covered by the _serialize_axis_result tests.