REF: Split up tests into modules & remove redundant testing

[adriangb]

• Split up tests into multiple modules
• Implement scikit-learn estimator checks on non-subclassed estimators
• Remove redundant tests and test parametrizations.
• Fix TST: Inconsistent failing for TestSampleWeights #43 by implementing a much better test.
• If pytest-xdist fails on CI, still fail the run but also try to run tests without xdist to get clean debugging info.

[codecov-commenter]

Codecov Report

Merging #56 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
- Coverage   99.51%   99.51%   -0.01%     
==========================================
  Files           3        3              
  Lines         415      413       -2     
==========================================
- Hits          413      411       -2     
  Misses          2        2              

Impacted Files	Coverage Δ
scikeras/wrappers.py	99.70% <ø> (-0.01%)	⬇️

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[adriangb]

@stsievert I know we went back and forth on checking that non-subclassed models can pass all tests. Here is proof that they do. As you suggested, the only test they fail is check_no_attributes_set_in_init. I added a separate tests specifically for that below.