Added working version of checking whether file is available for access

[mstfbl]

Recently AutoFitRegressionTest() and AutoFitBinaryTest() have been failing occasionally on our MachineLearning-Full CI builds with the error:

System.IO.IOException : The process cannot access the file 'D:\a\1\s\bin\AnyCPU.Debug\Microsoft.ML.AutoML.Tests\netcoreapp2.1...dataset' because it is being used by another process.

Stack trace
at System.IO.FileStream.ValidateFileHandle(SafeFileHandle fileHandle)
at System.IO.FileStream.CreateFileOpenHandle(FileMode mode, FileShare share, FileOptions options)
at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options)
at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share)
at Microsoft.ML.AutoML.TextFileSample.CreateFromFullFile(String path) in D:\a\1\s\src\Microsoft.ML.AutoML\ColumnInference\TextFileSample.cs:line 76
...

(Specific errors for AutoFitRegressionTest and AutoFitBinaryTest from CI builds)

This indicates that the .dataset file that is trying to be reached still has a lock on it, and is not yet ready for accessing. The current method of checking whether this file is ready to be accessed (checking new FileInfo(dataFile).Length > 0 is not sufficient.

The fix below remedies this by obtaining these dataset files from the local test/data path, and thereby eliminating the possibility of download locks. This fix also does the same for other datasets that are available locally that were being downloaded from a commit before, so that similar file lock issues for those datasets can be avoided as well.

[mstfbl]

With this build in the draft testing PR #4893, I have tested my changes with AutoFitRegressionTest() and AutoFitBinaryTest() running 100 iterations. These tests that used to hang are not not hanging anymore.

[frank-dong-ms-zz]

[justinormont]

LGTM