Consider using fixtures in tests

[astrojuanlu]

There are a lot of repeated code in the tests, mainly sample data for orbits. I have been experimenting with pytest fixtures lately and I think they are a great idea. They can even be parametrized, which might help repeating the same test with different kinds of orbits.

https://docs.pytest.org/en/3.8.0/fixture.html

[shreyasbapat]

Working on #451 I remembered that I may have left this issue half solved. I will search for the tests where I can use fixtures. Please let me know if you have any modules in mind.

[astrojuanlu]

While reviewing this, I realized that perhaps we could take https://github.com/poliastro/poliastro/blob/master/src/poliastro/tests/tests_twobody/test_angles.py and replace for loops with pytest.mark.parametrize. And also remove this leftover print:

poliastro/src/poliastro/tests/tests_twobody/test_angles.py

Line 121 in 5dfc7a1

print(M, ecc, M.value, ecc.value)

[astrojuanlu]

I don't see many places to replace current tests with fixtures. I imagine we could think about cases to improve or expand the tests by testing more kinds of orbits, but I am not sure whether pytest.mark.parametrize would be more suitable to the task.

In fact, I am going to close this issue, because it's "a solution in search of a problem" and might push us in the wrong direction.