MAINT use sklearn estimator tag in our test suite

[glemaitre]

Make use of the scikit-learn estimator tags to run our test suite.

[codecov]

Codecov Report

Merging #577 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #577      +/-   ##
==========================================
+ Coverage   98.85%   98.87%   +0.02%     
==========================================
  Files          84       84              
  Lines        5151     5156       +5     
==========================================
+ Hits         5092     5098       +6     
+ Misses         59       58       -1

Impacted Files	Coverage Δ
...rototype_selection/_instance_hardness_threshold.py	100% <100%> (ø)	⬆️
...mpling/_prototype_generation/_cluster_centroids.py	100% <100%> (ø)	⬆️
...rototype_selection/_condensed_nearest_neighbour.py	100% <100%> (ø)	⬆️
...n/under_sampling/_prototype_selection/_nearmiss.py	98.76% <100%> (+0.03%)	⬆️
...ling/_prototype_selection/_random_under_sampler.py	100% <100%> (ø)	⬆️
..._prototype_selection/_edited_nearest_neighbours.py	100% <100%> (ø)	⬆️
...rototype_selection/_neighbourhood_cleaning_rule.py	100% <100%> (ø)	⬆️
imblearn/utils/estimator_checks.py	96.9% <100%> (+0.16%)	⬆️
...nder_sampling/_prototype_selection/_tomek_links.py	100% <100%> (ø)	⬆️
imblearn/over_sampling/_random_over_sampler.py	100% <100%> (ø)	⬆️
... and 1 more

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