Add tests to verify that Sensei's templates are available in the site editor

[donnapep]

Is your feature request related to a problem? Please describe

https://github.com/Automattic/sensei/issues/7460 was the result of a regression in https://github.com/Automattic/sensei/pull/7402. There was no test coverage that checked to ensure Sensei's email template was available in the site editor, so adding some could help mitigate any future issues.

Describe the solution you'd like

We should add test coverage to ensure that Sensei's templates (lesson, quiz, email) are available in the site editor.