Tests emit a warning when run

We should also start running tests with -Werror flag.

=================================================================================== warnings summary ===================================================================================
pymablock/tests/test_block_diagonalization.py::test_check_diagonal_h_0_A
pymablock/tests/test_block_diagonalization.py::test_check_diagonal_h_0_B
  /home/anton/Bot/coding/pymablock/pymablock/block_diagonalization.py:1064: UserWarning: Cannot confirm that the unperturbed Hamiltonian diagonal, which is required if ``solve_sylvester`` is not provided. The algorithm will assume that it is diagonal.
    warn(

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html