CpuMath Enhancement: Add unit tests for software fallbacks of hardware intrinsics

[briancylui]

Style changes needed to solve part of #823

Details

• Add unit tests for software fallback implementations, particularly for MatTimesSrc.

Reference:

• The software fallback implementations are in src\Microsoft.ML.CpuMath\CpuMathUtils.netcoreapp.cs, encapsulated in the following switching paradigm:

if (Avx.IsSupported)
{
    // AvxIntrinsics
}
else if (Sse.IsSupported)
{
    // SseIntrinsics
}
else
{
    // Software fallback
}

• Unit tests are implemented in test\Microsoft.ML.CpuMath.UnitTests.netcoreapp/UnitTests.cs
• To turn on the software fallback implementation in unit tests, you may have to turn off the environment variable COMPlus_FeatureSimd.

[danmoseley]

Covered by #1156

[danmoseley]

Covered by #1156