Rename tests from multiIndex to multiindex.

[bdice]

Small PR for improving consistency in cuDF Python test names. Follows up from #10729 (comment). cc: @galipremsagar

[codecov]

Codecov Report

Merging #10732 (e724707) into branch-22.06 (94a5d41) will increase coverage by 0.00%.
The diff coverage is 91.90%.

❗ Current head e724707 differs from pull request most recent head de72c02. Consider uploading reports for the commit de72c02 to get more accurate results

@@              Coverage Diff              @@
##           branch-22.06   #10732   +/-   ##
=============================================
  Coverage         86.38%   86.39%           
=============================================
  Files               142      142           
  Lines             22334    22302   -32     
=============================================
- Hits              19294    19268   -26     
+ Misses             3040     3034    -6     

Impacted Files	Coverage Δ
python/cudf/cudf/_fuzz_testing/json.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/index.py	92.31% <ø> (ø)
python/cudf/cudf/utils/gpu_utils.py	50.00% <0.00%> (-4.29%)	⬇️
python/cudf/cudf/utils/utils.py	90.35% <ø> (+0.06%)	⬆️
python/cudf/cudf/core/df_protocol.py	88.48% <33.33%> (ø)
python/cudf/cudf/core/algorithms.py	90.47% <50.00%> (ø)
python/cudf/cudf/core/subword_tokenizer.py	75.00% <50.00%> (ø)
python/cudf/cudf/core/column/numerical.py	96.17% <66.66%> (ø)
python/cudf/cudf/testing/_utils.py	93.98% <88.88%> (+0.13%)	⬆️
python/cudf/cudf/core/frame.py	93.41% <89.47%> (-0.26%)	⬇️
... and 27 more

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[bdice]

@gpucibot merge