Testing: JSON variable testing improvements

[Fahad Heylaal]

When variables of json type are created in features, their values are defined as strings (stringified JSON).

Testing fails when assertions are provided in strings, because tester treats it as object/array (depending on JSON).

This can be fixed here with another case for JSON type: https://github.com/featurevisor/featurevisor/blob/main/packages/core/src/tester/testFeature.ts#L92-L98