Test/add xfel calibrate cli tests

Description

This MR adds in some basic tests for the CLI in xfel_calibrate. Done since I want to edit the CLI a bit for SQS Moonshot but the CLI is kinda spaghetti so I'm worried about breaking things and not realising it.

How Has This Been Tested?

Locally, tested by breaking code and checking that the tests fail.

Relevant Documents (optional)

Part of SQS Moonshot, see:

• https://git.xfel.eu/gitlab/moonshot/sqs/issues/13
• https://git.xfel.eu/gitlab/moonshot/sqs/issues/19

Types of changes

• Test (additional or refactored tests)

Checklist:

• My code follows the code style of this project.

Reviewers

@calibration