Fix several test and benchmark issues related to bitmask allocations.

[nvdbaranec]

I encountered a crash when trying to use the data coming out of create_random_table. The code was not generating buffers padded to 64 bytes, which was expected by the column utilities code. I made several changes to address this:

• Fix the core issue itself: make the bitmask buffers coming out of create_random_table conform to the cudf standard.

• Changed bitmask_to_host in column_utilites.cu to only return the number of bitmask words necessary to represent the number of rows in the column. Previously it was returning all the padding words as well, which was meaningless.

• Fixed an issue in test code that wasn't using cudf::test::validate_host_masks when it should have been.

[codecov]

Codecov Report

Merging #9521 (85c264b) into branch-21.12 (ab4bfaa) will decrease coverage by 0.11%.
The diff coverage is 0.00%.

@@               Coverage Diff                @@
##           branch-21.12    #9521      +/-   ##
================================================
- Coverage         10.79%   10.67%   -0.12%     
================================================
  Files               116      117       +1     
  Lines             18869    19714     +845     
================================================
+ Hits               2036     2104      +68     
- Misses            16833    17610     +777     

Impacted Files	Coverage Δ
python/cudf/cudf/__init__.py	0.00% <0.00%> (ø)
python/cudf/cudf/_lib/__init__.py	0.00% <ø> (ø)
python/cudf/cudf/core/_base_index.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/categorical.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/column.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/datetime.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/lists.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/numerical.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/string.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/timedelta.py	0.00% <0.00%> (ø)
... and 87 more

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[nvdbaranec]

@gpucibot merge