Modify dotnet-test to run testRunner for all tfms

Fixes #2506

TestAssets/TestProjects/ProjectsWithTests/NetCoreAppOnlyProject/GivenThatIWantSomeFakeTests.cs

@@ -0,0 +1,22 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using Xunit;
+
+namespace FakeTests
+{
+    public class GivenThatIWantSomeFakeTests
+    {
+        [Fact]
+        public void It_succeeds()
+        {
+            Assert.True(true);
+        }
+
+        [Fact]
+        public void It_also_succeeds()
+        {
+            Assert.True(true);
+        }
+    }
+}