constructor(toast) {
        super(ShowToastEventName, {
            composed: true,
            cancelable: true,
            bubbles: true,
            detail: toast
        });
    }
For the original lightning/platformShowToastEvent mock that comes by default with
@salesforce/sfdx-lwc-jest, see:
https://github.com/salesforce/sfdx-lwc-jest/blob/master/src/lightning-stubs/platformShowToastEvent/platformShowToastEvent.js