Add tests on lambda_t, low sigma values and guided functions

[kilianFatras]

What does this PR do?

I have added some tests on conditional flow matcher classes. Namely, I am now testing lambda_t, low sigma values and guided functions with return_noise.

Before submitting

• [x] Did you make sure title is self-explanatory and the description concisely explains the PR?
• [x] Did you make sure your PR does only one thing, instead of bundling different changes together?
• [x] Did you list all the breaking changes introduced by this pull request?
• [x] Did you test your PR locally with pytest command?
• [x] Did you run pre-commit hooks with pre-commit run -a command?

Did you have fun?

Make sure you had fun coding 🙃

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (e96c52c) 35.81% compared to head (3f08702) 35.90%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #80 +/- ## ========================================== + Coverage 35.81% 35.90% +0.09% ========================================== Files 67 67 Lines 7419 7419 ========================================== + Hits 2657 2664 +7 + Misses 4762 4755 -7 ```

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