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.