Improve 'Test using a Mock implementation' challenge episode 3.3

[mhagdorn]

The 'Test using a Mock implementation' challenge needs a few improvements:

Tasks

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1. The analyse_data function has not yet been fully refactored, it still contains the plot production and doesn't return anything
2. Mention that the failing of the test is expected and that we will adapt the analyse_data function to return an array in the next episode
3. Make it explicit that you should create a new test module in the test folder called test_compute_data.py, because the function we are testing is in compute_data.py

[svenvanderburg]

We should mention that the failing of the test is expected and that we will adapt the analyse_data function to return an array in the next episode: https://carpentries-incubator.github.io/python-intermediate-development/34-code-refactoring/index.html#exercise-write-regression-tests