Add support for source-build (dotnet/extensions#1740)

\n\nCommit migrated from dotnet/extensions@ad4067b

Author: natemcmaster

src/Testing/Directory.Build.props

@@ -0,0 +1,8 @@
+<Project>
+  <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory)..\, Directory.Build.props))\Directory.Build.props" />
+
+  <PropertyGroup>
+    <!-- These projects depend on a 3rd party source -->
+    <ExcludeFromSourceBuild>true</ExcludeFromSourceBuild>
+  </PropertyGroup>
+</Project>