Need infrastructure to run tests in varying hardware intrinsic modes

Increasingly we are taking changes that use hardware intrinsics to accelerate parts of CoreFX. Without special care, our testing will only ever cover the AVX2 (or AVX) path, not the software path and not any SSE path.

We need a way to easily cause selected tests to re-run in each mode so every codepath is exercised.

In the ML.NET repo they repeat relevant tests under each of regular, no AVX, and no AVX/SSE using RemoteExec: https://github.com/dotnet/machinelearning/blob/d65af0f755b9cd5739693004f1a8a8964ce23d8a/test/Microsoft.ML.CpuMath.UnitTests.netcoreapp/UnitTests.cs#L140

We can do something similar, but centralize the magic environment variables and offer a flag on RemoteExec that tests can set to enumerate over them.

cc @tannergooding @jkotas @Anipik