[tests] Set XamarinMacFrameworkRoot as an environment variable when r…

…unning MSBuild tests.

Set XamarinMacFrameworkRoot as an environment variable when running MSBuild
tests, because we have command-line tools that needs it (bgen), and only
passing it as a MSBuild parameter doesn't reach the environment (and any
subprocesses).

tests/common/mac/ProjectTestHelpers.cs

@@ -236,6 +236,7 @@ public static string BuildProject (string csprojTarget, bool isUnified, bool sho
 			Environment.SetEnvironmentVariable ("TargetFrameworkFallbackSearchPaths", rootDirectory + "/Library/Frameworks/Mono.framework/External/xbuild-frameworks");
 			Environment.SetEnvironmentVariable ("MSBuildExtensionsPathFallbackPathsOverride", rootDirectory + "/Library/Frameworks/Mono.framework/External/xbuild");
 			Environment.SetEnvironmentVariable ("XAMMAC_FRAMEWORK_PATH", rootDirectory + "/Library/Frameworks/Xamarin.Mac.framework/Versions/Current");
+			Environment.SetEnvironmentVariable ("XamarinMacFrameworkRoot", rootDirectory + "/Library/Frameworks/Xamarin.Mac.framework/Versions/Current");
 
 			// This is to force build to use our mmp and not system mmp
 			StringBuilder buildArgs = new StringBuilder ();