Introduce DotNetBuildSkipTests control

[mmitche]

See https://microsoft-my.sharepoint.com/:w:/p/mmitche/EdchH8A4okpOoKhkki1sJEYB0LyAaaCyX5_6A887gL744Q?e=JERvht for full design:

When “true”, the build should skip building test projects.
“true” is essentially the default behavior for source build today. This is essentially equivalent to ExcludeFromBuild being set to true when DotNetBuildSkipTests == true and Arcade’s IsTestProject or IsTestUtilityProject is true.

IIRC, present at outer repo and inner repo