Better use of pytest

[purva-thakre]

After #196 is fixed, make changes to how pytest is used in toqito/tests.

Things to change:

• Avoid redefining the same thing in different tests/modules.

More things to be added to the above checklist later.

https://pytest.org/en/7.4.x/explanation/goodpractices.html
https://emimartin.me/pytest_best_practices

• toqito/channel_metrics #333
• toqito/channel_ops #336
• toqito/channel_props
• toqito/channels
• toqito/helper
• toqito/matrices
• toqito/matrix_ops
• toqito/matrix_props
• toqito/measurement_ops
• toqito/measurement_props
• toqito/nonlocal_games
• toqito/perms
• toqito/rand
• toqito/state_metrics
• toqito/state_ops
• toqito/state_opt
• toqito/state_props
• toqito/states

[purva-thakre]

After #217 is completed, go back through all the tests in the test module and remove np.testing.

We want to use pytest for testing as some of the tests covered via np.testing were caught as missing by pytest.

isort is currently skipping tests. It might be better to change this later.

[purva-thakre]

Closing this as #488 has more details on what to do.