Rename strings/array_tests.cu to strings/array_tests.cpp

[davidwendt]

This PR renames cpp/tests/strings/array_tests.cu to .cpp file and cleans up several test cases. The gtests that required a .cu extension called the cudf::strings::detail::scatter function directly. This was created when cudf::scatter had not yet supported strings columns. Changing this to just calling cudf::scatter allows removing device code dependencies.

Also, the GatherTooBig test case was taking over 16s to run. Too much time was spent building fake data to just verify an exception. This PR fixes the fake data (which is never actually read) to speed up it's creation.

Finally, this PR includes removing the hash_string.cu since it is redundant with the hashing/hash_test.cpp which already covers strings inputs. The hash_string.cu had been created before strings support was added to cudf::hash.

[codecov]

Codecov Report

Merging #9480 (52a1163) into branch-21.12 (ab4bfaa) will decrease coverage by 0.14%.
The diff coverage is n/a.

❗ Current head 52a1163 differs from pull request most recent head 2d38f7b. Consider uploading reports for the commit 2d38f7b to get more accurate results

@@               Coverage Diff                @@
##           branch-21.12    #9480      +/-   ##
================================================
- Coverage         10.79%   10.64%   -0.15%     
================================================
  Files               116      117       +1     
  Lines             18869    19363     +494     
================================================
+ Hits               2036     2062      +26     
- Misses            16833    17301     +468     

Impacted Files	Coverage Δ
python/dask_cudf/dask_cudf/sorting.py	92.90% <0.00%> (-1.21%)	⬇️
python/cudf/cudf/io/csv.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/orc.py	0.00% <0.00%> (ø)
python/cudf/cudf/__init__.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/frame.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/index.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/parquet.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/series.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/reshape.py	0.00% <0.00%> (ø)
python/cudf/cudf/utils/dtypes.py	0.00% <0.00%> (ø)
... and 30 more

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[harrism]

@gpucibot merge