[Multithreaded MSBuild] Design and run tests

[AR-May]

Design and implement tests to find rare race conditions in the multithreaded MSBuild implementation. Fix found issues.

Specific scenarios of interest:

• simple console app (e.g. dotnet new console)
  • useful to help track the end-to-end perf of the 'getting started' experience
• MSBuild repo itself
  • We do fairly non-trivial things with custom, local tasks, etc
• Orchard Code
  • representative of large-ish .NET SDK-style workspaces
• A non-SDK-style workspace/project build
  • should show great impact of MT because most Tasks are provided by MSBuild - earlier visibility into potential gains
• A workspace that uses static graph
  • Shouldn't impact execution, which is what multithreaded is mostly concerned with, but a good thing to have verified
• A workspace that uses the project cache plugin
  • Shouldn't impact multithreaded mode because the project cache plugin entirely replaces calls to build a specific project, but a good thing to have verified