Add DataFrame.IO tests with separators in data

[dakersnar]

#5647 is no longer an issue, but there are no existing unit tests that confirm this behavior. Adding them here.

[codecov]

Codecov Report

Merging #6301 (d72df26) into main (f117cd8) will increase coverage by 0.11%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6301      +/-   ##
==========================================
+ Coverage   68.39%   68.50%   +0.11%     
==========================================
  Files        1144     1170      +26     
  Lines      244991   247065    +2074     
  Branches    25411    25669     +258     
==========================================
+ Hits       167558   169261    +1703     
- Misses      70775    71067     +292     
- Partials     6658     6737      +79     

Flag	Coverage Δ
Debug	68.50% <100.00%> (+0.11%)	⬆️
production	62.94% <ø> (+0.10%)	⬆️
test	89.00% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...Microsoft.Data.Analysis.Tests/DataFrame.IOTests.cs	98.79% <100.00%> (+0.20%)	⬆️
...Microsoft.ML.TorchSharp/Utils/DefaultDictionary.cs	0.00% <0.00%> (-30.62%)	⬇️
test/Microsoft.ML.AutoML.Tests/DatasetUtil.cs	81.06% <0.00%> (-17.93%)	⬇️
...luators/Metrics/MulticlassClassificationMetrics.cs	89.47% <0.00%> (-10.53%)	⬇️
test/Microsoft.ML.AutoML.Tests/AutoFitTests.cs	76.32% <0.00%> (-9.43%)	⬇️
test/Microsoft.ML.Tests/TextClassificationTests.cs	93.75% <0.00%> (-6.25%)	⬇️
src/Microsoft.ML.Maml/MAML.cs	25.38% <0.00%> (-1.53%)	⬇️
.../Microsoft.ML.Vision/ImageClassificationTrainer.cs	92.13% <0.00%> (-0.12%)	⬇️
...ML.TorchSharp/NasBert/TextClassificationTrainer.cs	87.73% <0.00%> (-0.05%)	⬇️
src/Microsoft.ML.SearchSpace/Parameter.cs	69.88% <0.00%> (ø)
... and 42 more

[dakersnar]

I'm closing this as I'm including and fleshing out these tests a bit more in a larger PR.