addTestId method
Adds value
to the prop key
(delimited with a single space).
Allows for an element to have multiple test IDs to prevent overwriting when cloning elements or components.
For use in a testing environment (when testMode is true).
Implementation
@override
void addTestId(String? value, {String key = defaultTestIdKey}) =>
throw UnimplementedError('@PropsMixin instances do not implement addTestId');