Test fix using breastcancel dataset and test cleanup

[frank-dong-ms-zz]

this PR is part of below PR, we decide to hold the text loader PR but this test fix can be merged separately:
#5275

2 fixes in this PR:

1. breastcancel dataset don't have header, fix tests using this dataset with option hasHeader from true to false
2. we already have dataset breastcancel so stop using hardcoded string "breast-cancer.txt"

[codecov]

Codecov Report

Merging #5292 into master will decrease coverage by 0.06%.
The diff coverage is 96.93%.

@@            Coverage Diff             @@
##           master    #5292      +/-   ##
==========================================
- Coverage   73.75%   73.68%   -0.07%     
==========================================
  Files        1022     1022              
  Lines      190360   190360              
  Branches    20472    20472              
==========================================
- Hits       140401   140272     -129     
- Misses      44442    44558     +116     
- Partials     5517     5530      +13     

Flag	Coverage Δ
#Debug	73.68% <96.93%> (-0.07%)	⬇️
#production	69.43% <ø> (-0.09%)	⬇️
#test	87.62% <96.93%> (ø)

Impacted Files	Coverage Δ
...L.Predictor.Tests/TestParallelFasttreeInterface.cs	0.00% <0.00%> (ø)
test/Microsoft.ML.TestFramework/TestCommandBase.cs	44.96% <66.66%> (ø)
...crosoft.ML.Core.Tests/UnitTests/TestEntryPoints.cs	98.43% <100.00%> (ø)
test/Microsoft.ML.Predictor.Tests/TestIniModels.cs	100.00% <100.00%> (ø)
...est/Microsoft.ML.Predictor.Tests/TestPredictors.cs	70.11% <100.00%> (ø)
...icrosoft.ML.TestFramework/DataPipe/TestDataPipe.cs	91.06% <100.00%> (ø)
...soft.ML.TestFramework/DataPipe/TestDataPipeBase.cs	73.85% <100.00%> (ø)
test/Microsoft.ML.Tests/OnnxConversionTest.cs	96.57% <100.00%> (ø)
...soft.ML.Tests/PermutationFeatureImportanceTests.cs	100.00% <100.00%> (ø)
test/Microsoft.ML.Tests/Scenarios/Api/TestApi.cs	94.16% <100.00%> (ø)
... and 32 more

[mstfbl]

I noticed that "breast-cancer-withheader.txt" does not have its own TextDataset breastCancerWithHeaderPipe in Datasets.cs, perhaps we can add that too here in the future.

machinelearning/test/Microsoft.ML.TestFrameworkCommon/Datasets.cs

Line 48 in a00a222

public static class TestDatasets