[MRG+1] Instance level common tests (#9019)

* start work on separating instance-level tests

* minor refactoring / fixes to work without tags

* add clone into check_supervised_y_2d estimator check (which made other checks fail)

* remove duplicate check_estimator_unfitted assert

* add issue reference to whatsnew entry

* added some clones, minor fixes from vene's review

* rename estimator arg to estimator_org to make a visible distinction before and after cloning.

* more renaming for more explicit clones

* org -> orig

* allclose, fix orig stuff

* don't use set_testing_parameters in the checks!

* minor fixes for allclose

* fix some test, add more tests on classes

* added the test using pickles.

* move assert_almost_equal_dense_sparse to utils.testing, rename to assert_allclose_sparse_dense, test it

* make assert_allclose_dense_sparse more stringent

* more allclose fixes

* run test_check_estimator on all estimators

* rename set_testing_parameters to set_checking_parameters so nose doesn't think it's a tests (and I don't want to import stuff from nose as we want to remove it)

* fix in set_checking_parameters so that common tests pass

* more fixes to assert_allclose_dense_sparse

* rename alg to clusterer, don't scream even though I really want to

* ok this is not a pretty strict test that runs check_estimator with and without fitting on an instance. I also check if ``fit`` is called on the instance that is passed.

* simplify test as they didn't help at all

* it works!!! omfg

* run check_estimator clone test only on one of the configs, don't run locally by default

* Add `slow_test` decorator and documentation

* run test_check_estimator only on some estimators

* fix diags in test for older scipy

* fix pep8 and shorten

* use joblib.hash for inequality check because the pickle state machine is weird