Add a section about testing structural directive

[dev054]

📚 Docs or angular.io bug report

Description

Similar to what we have in https://angular.io/guide/testing-attribute-directives, I want to suggest the creation of a section about testing structural directive.

Related to #32340.

[jelbourn]

@aikidave I think that the existing content under Testing Attribute Directives is actually just a plain "Testing Directives" content. Off the top of my head, I don't think there's anything special to testing regular directives vs. structural directives. We could, though, add an example for testing a structural directive.

[kapunahelewong]

Capturing this here. This issue's point could potentially tie into the info on template type checking (TTC) in the structural directives guide. That info also applies to attribute directives. In this comment, @jelbourn mentions moving the TTC info into a more general location. Maybe together (if relevant) we would have enough material to constitute a new document. Just thinking out loud here.