TST: Increase code coverage to 90%

[Gui-FernandesBR]

For a python library like ours, a 90% coverage threshold should be the ideal.
It should already consider the slow tests as well.

Coverage is not the only metric that matters, but it is the best one.

[Gui-FernandesBR]

As of today, we have a cc of 87% when considering the slow tests.

Unfortunately, when we exclude the slow tests this number reduced to only 76%.

Ideally, given that RocketPy is a library, code coverage should be of 99.9% with slow tests and 90% without the slow tests