[msbuild] improved lookup of Xamarin.Forms.targets in integration tests

Context: https://devdiv.visualstudio.com/0bdbc590-a062-4c3f-b0f6-9383f67865ee/_build/index?buildId=1717939

It looks like the test assemblies are now copied to a build staging
directory on VSTS.

We can rely on `BUILD_STAGINGDIRECTORY` to be set when running in
VSTS, and use an alternative place to find `Xamarin.Forms.targets`.

I also made the tests abort earlier if they can't find
`Xamarin.Forms.targets`.

Author: jonathanpeppers

Xamarin.Forms.Xaml.UnitTests/MSBuild/MSBuildTests.cs

@@ -13,7 +13,7 @@ namespace Xamarin.Forms.Xaml.UnitTests
 	[TestFixture]
 	public class MSBuildTests
 	{
-		static readonly string XamarinFormsTargets = Path.Combine ("..", "..", "..", "..", "..", ".nuspec", "Xamarin.Forms.targets");
+		static readonly string XamarinFormsTargets = Path.Combine (".nuspec", "Xamarin.Forms.targets");
 
 		static readonly string [] references = new []
 		{
@@ -120,7 +120,23 @@ XElement NewProject (bool sdkStyle)
 
 			if (!sdkStyle)
 				project.Add (NewElement ("Import").WithAttribute ("Project", @"$(MSBuildBinPath)\Microsoft.CSharp.targets"));
-			project.Add (NewElement ("Import").WithAttribute ("Project", XamarinFormsTargets));
+
+			var xamarinFormsTargets = Path.Combine (tempDirectory, "..", "..", "..", "..", "..", XamarinFormsTargets);
+			if (!File.Exists (xamarinFormsTargets)) {
+				//NOTE: VSTS we can use an environment variable
+				//	https://docs.microsoft.com/en-us/vsts/build-release/concepts/definitions/build/variables?view=vsts&tabs=batch#buildsourcesdirectory
+				var sourcesDirectory = Environment.GetEnvironmentVariable ("BUILD_SOURCESDIRECTORY");
+				if (!string.IsNullOrEmpty (sourcesDirectory)) {
+					xamarinFormsTargets = Path.Combine (sourcesDirectory, XamarinFormsTargets);
+					if (!File.Exists (xamarinFormsTargets)) {
+						Assert.Fail ("Unable to find Xamarin.Forms.targets at path: " + xamarinFormsTargets);
+					}
+				} else {
+					Assert.Fail ("Unable to find Xamarin.Forms.targets at path: " + xamarinFormsTargets);
+				}
+			}
+
+			project.Add (NewElement ("Import").WithAttribute ("Project", Path.GetFullPath (xamarinFormsTargets)));
 			return project;
 		}