Add fixture in common tests and speed them up

[chkoar]

In tests it could be ok to have copy/paste code but I believe that in common estimator checks we could remove the boilerplate of datasets generation for each test by creating a common fixture. Apart from that we could reduce the number of instances of the dataset(s) in order to speed up the execution of the commont tests. The later could be measured, though.

[glemaitre]

I am +1 for using pytest fixture. We should just avoid that they are too magical (e.g. hidden inside the conf file). But we should try to make use of them. It will also accelerate the testing time most probably.

[awinml]

I would like to work on this issue. I will create a common fixture in imblearn/utils/estimator_checks.py and remove the dataset generation from all the necessary tests.