NavigationMixin = (Base) => {
    return class extends Base {
        [Navigate](pageReference, replace) {
            mockNavigate({ pageReference, replace });
        }
        [GenerateUrl](pageReference) {
            mockGenerate({ pageReference });
            return new Promise((resolve) => resolve('https://www.example.com'));
        }
    };
}
For the original lightning/navigation mock that comes by default with
@salesforce/sfdx-lwc-jest, see:
https://github.com/salesforce/sfdx-lwc-jest/blob/master/src/lightning-stubs/navigation/navigation.js