Add test cover for ScheduledPublishWidget

Problem/Motivation

Issue 🐛 Replace t() calls with $this->t() in classes Fixed introduced an error due to a reference of $this in a static member. This was missed by the CI because there is no cover for the ScheduledPublishWidget class.

Proposed resolution

Add a new test to verify the ScheduledPublishWidget class.

This could either be a test that covers just the class, or a functional test that simulates creating and scheduling a node.