Sort by index in groupby tests more consistently

[shwina]

Pandas sorts the result of a groupby by its index by default, whereas we do not. For testing purposes, we currently call groupby() with sort=True in many of our tests. But some tests (notably newer ones) don't do that. These tests have been passing silently. Until now.

• adds a assert_groupby_results_equal helper in test_groupby.py, which sorts its inputs by index before comparing them
• removes the manual sort=True we are doing in each of our groupby tests

[galipremsagar]

LGTM

[codecov]

Codecov Report

Merging #7802 (b7a6da1) into branch-0.19 (7871e7a) will increase coverage by 0.78%.
The diff coverage is n/a.

❗ Current head b7a6da1 differs from pull request most recent head 53e5d7f. Consider uploading reports for the commit 53e5d7f to get more accurate results

@@               Coverage Diff               @@
##           branch-0.19    #7802      +/-   ##
===============================================
+ Coverage        81.86%   82.64%   +0.78%     
===============================================
  Files              101      103       +2     
  Lines            16884    17566     +682     
===============================================
+ Hits             13822    14518     +696     
+ Misses            3062     3048      -14     

Impacted Files	Coverage Δ
python/cudf/cudf/utils/dtypes.py	83.44% <0.00%> (-6.08%)	⬇️
python/cudf/cudf/core/column/lists.py	87.32% <0.00%> (-4.08%)	⬇️
python/cudf/cudf/core/column/decimal.py	92.68% <0.00%> (-2.19%)	⬇️
python/dask_cudf/dask_cudf/backends.py	87.50% <0.00%> (-2.13%)	⬇️
python/cudf/cudf/core/groupby/groupby.py	92.41% <0.00%> (-1.04%)	⬇️
python/cudf/cudf/core/column/column.py	87.61% <0.00%> (-0.15%)	⬇️
python/cudf/cudf/utils/utils.py	85.36% <0.00%> (-0.07%)	⬇️
python/cudf/cudf/io/feather.py	100.00% <0.00%> (ø)
python/cudf/cudf/utils/ioutils.py	78.71% <0.00%> (ø)
python/cudf/cudf/comm/serialize.py	0.00% <0.00%> (ø)
... and 46 more

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[kkraus14]

@gpucibot merge

[kkraus14]

rerun tests