Fix cudf::merge gtest for dictionary columns

[davidwendt]

The cudf::merge API expects the key columns to be sorted. This means that if null rows are included, these null entries should all appear either at beginning or at the end of the column depending on the null_order for the sort. The MergeDictionaryTest.WithNull gtest placed null rows in the middle of the column. The expected results should also have included null entries at the beginning or the end.

This PR also includes an extra test for checking merge results are consistent with the sort parameters cudf::order and cudf::null_order. This test also includes a larger number of rows to ensure thrust::merge requires more than one tile/block in its runtime logic.

[GPUtester]

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[codecov]

Codecov Report

Merging #6942 (dc97685) into branch-0.18 (917759b) will increase coverage by 0.46%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.18    #6942      +/-   ##
===============================================
+ Coverage        81.57%   82.03%   +0.46%     
===============================================
  Files               96       96              
  Lines            15912    16282     +370     
===============================================
+ Hits             12980    13357     +377     
+ Misses            2932     2925       -7     

Impacted Files	Coverage Δ
python/cudf/cudf/io/feather.py	100.00% <0.00%> (ø)
python/cudf/cudf/comm/serialize.py	0.00% <0.00%> (ø)
python/cudf/cudf/_fuzz_testing/io.py	0.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/_version.py	0.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/tests/test_csv.py	100.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/tests/test_orc.py	100.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/tests/test_json.py	100.00% <0.00%> (ø)
...ython/dask_cudf/dask_cudf/io/tests/test_parquet.py	100.00% <0.00%> (ø)
python/cudf/cudf/core/dataframe.py	91.08% <0.00%> (+<0.01%)	⬆️
python/cudf/cudf/utils/applyutils.py	98.74% <0.00%> (+0.02%)	⬆️
... and 36 more

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