Simplify trigger email notification test

What does this MR do?

This MR simplifies the trigger email notification test.

It's better to have the implementation details encapsulated in a method that is called by the test than having all of it inside the test itself. This way the it block gets cleaner and more focused.

The refactored test was executed in the following job, and it has passed https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/401482308.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team