Adding initial Helix support

[stevenbrix]

Testing in Helix

This PR enables running our DRT tests on Helix. I'd recommend seeing the official Helix readme if you are interested in some of the general Helix concepts. I'll briefly outline what we are doing that is a bit unique:

1. Helix has built-in support for running xUnit tests. Since we are not using xUnit, we have to manually setup our machines so that they work with QualityVault and STI. During the build, we create a payload directory that contains the infrastructure we need. A single project (in our case DrtXaml) is responsible for creating this directory (see instances of the MSBuild property CreateTestPayload).
2. After the build is done, we utilize Arcade's AfterSolutionBuild.targets extension point to finish creating the rest of the payload if the -test parameter is passed to the build. Here we add the just built DRTs and if -ci was not passed into to the build, run the tests.
3. Helix allows you to specify a HelixCorrelationPayload directory, where this directory gets deployed to the Helix machine, and is made available in your various helix commands with the HELIX_CORRELATION_PAYLOAD environment variable. We use the payload directory created described above.
4. Helix and Azure Pipelines can report xUnit logs, so QualityVault was updated to produce an xUnit compatible log. We then copy that log to a known location for it to be picked up. This location can be in any subfolder of the HelixWorkItem working directory.

How we are running tests

1. Currently, the HelixQueues that we selected are Windows.10.Amd64.Open;Windows.7.Amd64.Open;Windows.10.Amd64.ServerRS5.Open. Essentially, this translates to: "Latest Windows 10", "Windows 7", and "Windows 10 RS5 Server" addition. This enables tests to run on some of the most interesting and important SKUs, without overloading the Helix servers and/or making CI runs take an unnecessarily long time to run.
2. In a similar fashion, we only run test passes for Release builds, mainly to save time and resources running duplicate tests in a near-identical environment.

Fixes #782

[rladuca]

Just marking NO MERGE until Preview 6 clears.

[stevenbrix]

I removed the no merge tag since the preview6 branch has snapped.

For more detailed information on some of the workarounds, see this comment:
#816 (comment)