Extend unit test coverage with `Hypothesis` in numerical tests

[edoaltamura]

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What should we add?

The hypothesis package could help enhance the unit test coverage in this library. It's a robust toolkit for testing algorithms like QSVC with edge cases and different random seeds automatically, without having to hard-wire test cases. The generated tests are reproducible and can be inspected individually with logs.

For more info on this testing library, read the manifesto.