Build script improvements

[adamralph]

• [x] enable all CPUs for msbuild (with nodereuse false)
• [x] log msbuild at detailed verbosity to a file in /artifacts/logs (verbosity overridable by ENV var)
• [x] turn down msbuild verbosity to minimum
• [x] turn down nuget verbosity to quiet (overridable by ENV var)
• [x] change test result output to /artifacts/tests