Create tests for checking if email is sent for new mentorship relation requests.

## Description
As a developer for backend api,
I need cover most part of the logic of backend,
so that I can be confident that the app works as it is supposed to.

We send email notifications when new mentorship relationship was created. We need to create tests to check the functionality of this specific function.

PR #276 can help to understand what you may have to use to complete this task

## Acceptance Criteria

### UPDATE [Required]
- [ ] Write test for checking if the mail was sent if the mentorship request is created.
- [ ] Write test for checking if the mail wasn't sent if the mentorship request wasn't created (Invalid).

### Definition of Done
- [ ] All the required items are completed.
- [ ] Approval by 1 mentor.

### Estimation
12 hours