[REVIEW] Reduce number of concatenate benchmark test cases

[trevorsm7]

The concatenate benchmark is taking an excessive amount of CI time because it sweeps a large range of parameters. Increasing the range multiplier from 4 to 8 will greatly reduce the number of test cases, which should alleviate the issue.

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[codecov]

Codecov Report

Merging #5743 into branch-0.15 will decrease coverage by 2.26%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.15    #5743      +/-   ##
===============================================
- Coverage        88.65%   86.38%   -2.27%     
===============================================
  Files               57       76      +19     
  Lines            10945    13036    +2091     
===============================================
+ Hits              9703    11261    +1558     
- Misses            1242     1775     +533     

Impacted Files	Coverage Δ
python/cudf/cudf/core/dtypes.py	87.71% <0.00%> (-9.21%)	⬇️
python/cudf/cudf/core/abc.py	91.30% <0.00%> (-3.15%)	⬇️
python/cudf/cudf/utils/ioutils.py	85.85% <0.00%> (-1.49%)	⬇️
python/dask_cudf/dask_cudf/sorting.py	93.07% <0.00%> (-1.05%)	⬇️
python/cudf/cudf/core/frame.py	90.19% <0.00%> (-0.98%)	⬇️
python/cudf/cudf/core/indexing.py	95.27% <0.00%> (-0.63%)	⬇️
python/cudf/cudf/core/multiindex.py	80.34% <0.00%> (-0.29%)	⬇️
python/cudf/cudf/core/column/numerical.py	94.33% <0.00%> (-0.05%)	⬇️
python/cudf/cudf/io/avro.py	81.81% <0.00%> (ø)
python/cudf/cudf/core/ops.py	77.58% <0.00%> (ø)
... and 55 more

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