Improve CSV writer tests

[vuule]

Closes #6836

Overall reduces test execution time and improves coverage:

• Replace around 2.5K test cases with multiple tests that only vary related options.
• Correctly verify the output with multicharacter line_terminator option (not supported by readers).
• Add a seed call before the random generator is used in one of the tests.
• Simplify a few tests by removing irrelevant comparisons.
• Use buffer output instead of file in affected tests (could be applied to many more).

[codecov]

Codecov Report

Merging #7851 (4d95cd2) into branch-0.20 (51336df) will decrease coverage by 0.36%.
The diff coverage is 88.34%.

❗ Current head 4d95cd2 differs from pull request most recent head cbfc64d. Consider uploading reports for the commit cbfc64d to get more accurate results

@@               Coverage Diff               @@
##           branch-0.20    #7851      +/-   ##
===============================================
- Coverage        82.88%   82.52%   -0.37%     
===============================================
  Files              103      103              
  Lines            17668    17300     -368     
===============================================
- Hits             14645    14277     -368     
  Misses            3023     3023              

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/__init__.py	100.00% <ø> (ø)
python/cudf/cudf/io/orc.py	86.80% <ø> (-0.10%)	⬇️
python/cudf/cudf/utils/cudautils.py	55.04% <25.00%> (-2.72%)	⬇️
python/cudf/cudf/core/column/numerical.py	94.41% <70.00%> (-0.02%)	⬇️
python/cudf/cudf/core/column/column.py	88.48% <71.42%> (-0.17%)	⬇️
python/dask_cudf/dask_cudf/backends.py	89.20% <80.00%> (-0.38%)	⬇️
python/cudf/cudf/core/dataframe.py	90.75% <83.33%> (-0.11%)	⬇️
python/cudf/cudf/utils/utils.py	89.47% <91.66%> (-0.04%)	⬇️
python/cudf/cudf/core/column/datetime.py	89.40% <100.00%> (-0.52%)	⬇️
python/cudf/cudf/core/column/timedelta.py	88.33% <100.00%> (-0.34%)	⬇️
... and 58 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4893259...cbfc64d. Read the comment docs.

[vuule]

rerun tests

[vuule]

rerun tests

[kkraus14]

@gpucibot merge