[xharness] Resolve relative project references using the referencing …

…project's directory. (#9252)

Otherwise it would end up being resolved from the current directory, which is
not dependable.

tests/xharness/Microsoft.DotNet.XHarness.iOS.Shared/Tasks/BuildProject.cs

@@ -87,6 +87,8 @@ List<string> GetNestedReferenceProjects (string csproj)
 				var fixPath = referenceProject.Replace ("\\", "/"); // do the replace in case we use win paths
 				result.Add (fixPath);
 				// get all possible references
+				if (!Path.IsPathRooted (fixPath))
+					fixPath = Path.Combine (Path.GetDirectoryName (csproj), fixPath);
 				result.AddRange (GetNestedReferenceProjects (fixPath));
 			}
 			return result;