message: Update tests/Directory.Build.targets
diff: Directory.Build.targets is automatically picked up and imported by
  Microsoft.Common.targets. This file needs to exist, even if empty so that
  files in the parent directory tree, with the same name, are not imported
  - instead. The import fairly late and most other props/targets will have
  + instead. They import fairly late and most other props/targets will have
  been imported beforehand. We also don't need to add ourselves to
  MSBuildAllProjects, as that is done by the file that imports us.
  -->
  