Introduce publishEvent() convenience method in TestContext

[sbrannen]

Status Quo

It has always been possible for a TestExecutionListener to publish an ApplicationEvent to the current test's ApplicationContext via testContext.getApplicationContext().publishEvent(new MyEvent()); however, that resulted in undesirable loading of the ApplicationContext in some cases.

Typically people only want to publish events if the ApplicationContext is currently available.

The testContext.hasApplicationContext() support introduced in gh-22756 makes it possible to only publish events if the ApplicationContext is currently available as follows.

if (testContext.hasApplicationContext()) {
	testContext.getApplicationContext().publishEvent(new MyEvent());
}

This issue aims to introduce a convenience method in the TestContext API that performs the above with lazy creation of the actual ApplicationEvent.

Deliverables

• Introduce publishEvent() convenience method in TestContext.