Piotrp msft/msbuild/incremental test (#3842)

* Incremental Testing - Part 1

* Fix 'clean' bug

* Remove --fallbacksource from TestAssetPackage restore.

build_projects/dotnet-cli-build/DotNetTest.cs

@@ -0,0 +1,54 @@
+using Microsoft.Build.Framework;
+using Microsoft.Build.Utilities;
+
+namespace Microsoft.DotNet.Cli.Build
+{
+    public class DotNetTest : DotNetTool
+    {
+        protected override string Command
+        {
+            get { return "test"; }
+        }
+
+        protected override string Args
+        {
+            get { return $"{GetConfiguration()} {GetXml()} {GetNoTrait()}"; }
+        }
+
+        public string Configuration { get; set; }
+
+        public string Xml { get; set; }
+
+        public string NoTrait { get; set; }
+
+        private string GetConfiguration()
+        {
+            if (!string.IsNullOrEmpty(Configuration))
+            {
+                return $"--configuration {Configuration}";
+            }
+
+            return null;
+        }
+
+        private string GetNoTrait()
+        {
+            if (!string.IsNullOrEmpty(Configuration))
+            {
+                return $"-notrait {NoTrait}";
+            }
+
+            return null;
+        }
+
+        private string GetXml()
+        {
+            if (!string.IsNullOrEmpty(Xml))
+            {
+                return $"-xml {Xml}";
+            }
+
+            return null;
+        }
+    }
+}