Remove build.cmd fallback to v4 version of MSBuild.exe
- we don't support .NET 4.0 and shouldn't build or test using the older
  target framework
- testing is particularly problematic since some unit tests fail due to
  .NET 4.0 quirks (legacy System.Uri behavior in particular)
- part 1 of fixes for #1960
Revert "Disable tests that are currently failing on CI machine"
- tests run fine as long as we avoid .NET 4.0 and back-compat quirks mode
- part 2 of fixes for #1960

This reverts commit 064e469915c3a3979c5cebadbdb9eb804c6e757f.