Determine Helix test coverage strategy

[jander-msft]

The Helix environment allows us to test on a multitude of variants of OSes, distros, and architectures. While we would want to get the widest possible coverage, we may want to be judicious on the size of the matrix depending on the reason the tests are running (e.g. PR builds, CI builds, scheduled builds, etc).

For reference, these are the supported OS matrixes for each in-support .NET version:

• https://github.com/dotnet/core/blob/main/release-notes/6.0/supported-os.md
• https://github.com/dotnet/core/blob/main/release-notes/7.0/supported-os.md
• https://github.com/dotnet/core/blob/main/release-notes/8.0/supported-os.md