Eternal: Testing

[David-Kreplin]

We want to improve our test coverage in the short term and require unit tests in the long term. Here are some things, that still need to be tested:

• [ ] Test QNNs with Pauli X & Y
• [ ] Test QNNs with Sampler
• [ ] Test QNNs with Estimator
• [ ] Examples for automatic backend selection

Also create a coverage report with

pytest --cov-report term-missing --cov=squlearn tests/

to see which lines aren't tested yet.

[MoritzWillmann]

We should have some sort of integration tests for scikit-learn pipelines and CV routines... see #347