Create assertIsApplicable() and assertIsNotApplicable() in widget base class

This is a follow-on from #3109923: Deprecation: add $defaultTheme - create FormWidgetBrowserTestBase class → which adds a new FormWidgetBrowserTestBase class.

In that base class we can define protected functions assertIsApplicable($widget, $type) and assertIsNotApplicable($widget, $type) to simplify the testIsApplicable function then the subclasses don't need to deal with DataDefinition at all.

Instead of the subclasses having to do $this->assertFalse($this->widget->isApplicable(DataDefinition::create('boolean'))); etc, this will now be in the helper function, and the subclasses just substitute $this->assertIsApplicable($this->widget, 'boolean'); etc.

Credit to TR for this idea