[REVIEW] Fix vectorizer tests by restoring sort behavior in groupby

[JohnZed]

cuDF group by now no longer performs a sort by default. This change just restores the old behavior in vectorizers by applying a sort to everything.

[codecov-io]

Codecov Report

Merging #3416 (41bcad0) into branch-0.18 (550121b) will increase coverage by 0.17%.
The diff coverage is 85.19%.

@@               Coverage Diff               @@
##           branch-0.18    #3416      +/-   ##
===============================================
+ Coverage        71.48%   71.66%   +0.17%     
===============================================
  Files              207      210       +3     
  Lines            16748    16945     +197     
===============================================
+ Hits             11973    12144     +171     
- Misses            4775     4801      +26     

Impacted Files	Coverage Δ
python/cuml/decomposition/incremental_pca.py	94.70% <ø> (ø)
python/cuml/dask/ensemble/base.py	19.69% <30.43%> (+0.36%)	⬆️
python/cuml/dask/cluster/kmeans.py	54.00% <33.33%> (ø)
python/cuml/ensemble/randomforestregressor.pyx	70.83% <44.44%> (ø)
python/cuml/dask/decomposition/base.py	39.53% <50.00%> (ø)
...ython/cuml/dask/ensemble/randomforestclassifier.py	30.00% <50.00%> (+0.51%)	⬆️
python/cuml/dask/ensemble/randomforestregressor.py	35.08% <50.00%> (+0.54%)	⬆️
python/cuml/dask/linear_model/linear_regression.py	59.09% <50.00%> (ø)
python/cuml/dask/linear_model/ridge.py	50.00% <50.00%> (ø)
...ython/cuml/dask/neighbors/kneighbors_classifier.py	22.33% <50.00%> (ø)
... and 27 more

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[dantegd]

@gpucibot merge