Implement E2E test

[lorerod]

Describe the issue Currently, the project doesn't count with a set of e2e tests, only unit tests for the mediator.

Describe the improvement you'd like Implement the e2e test for one first scenario: The LTFU flow happy path

Describe alternatives you've considered None.

Tasks:

• [x] Implement e2e LTFU flow
• [x] Add e2e to CI pipeline

[andrablaj]

@lorerod closing this issue as it was moved to Done in the FG board. Great job!