[REVIEW] Speed up test_linear_model

[wphicks]

Improve runtime of unit tests in test_linear_model by about a factor of 6
Remove redundant tests that exercise the same sections of code with trivial or useless variations
Add caching for sample regression dataset creation

Partial fix for #3045

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[wphicks]

rerun tests

[codecov-io]

Codecov Report

Merging #3075 into branch-0.17 will increase coverage by 0.84%.
The diff coverage is 89.28%.

@@               Coverage Diff               @@
##           branch-0.17    #3075      +/-   ##
===============================================
+ Coverage        58.39%   59.23%   +0.84%     
===============================================
  Files              143      142       -1     
  Lines             8897     8966      +69     
===============================================
+ Hits              5195     5311     +116     
+ Misses            3702     3655      -47     

Impacted Files	Coverage Δ
python/cuml/common/import_utils.py	61.95% <0.00%> (-2.42%)	⬇️
python/cuml/dask/solvers/cd.py	54.54% <ø> (+2.16%)	⬆️
python/cuml/thirdparty_adapters/adapters.py	88.44% <95.74%> (+6.20%)	⬆️
...l/_thirdparty/sklearn/preprocessing/_imputation.py	62.50% <100.00%> (-4.05%)	⬇️
python/cuml/thirdparty_adapters/__init__.py	100.00% <100.00%> (ø)
python/cuml/pytest_benchmarks/test_bench.py	100.00% <0.00%> (ø)
python/cuml/common/array.py	97.72% <0.00%> (+0.01%)	⬆️
python/cuml/metrics/_ranking.py	98.61% <0.00%> (+0.03%)	⬆️
python/cuml/preprocessing/encoders.py	95.18% <0.00%> (+0.07%)	⬆️
...on/cuml/_thirdparty/sklearn/preprocessing/_data.py	63.45% <0.00%> (+0.12%)	⬆️
... and 35 more

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[dantegd]

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