Modify dotnet-test to run testRunner for all tfms

Fixes #2506

TestAssets/TestProjects/ProjectsWithTests/MultipleFrameworkProject/GivenThatIWantSomeFakeTests.cs

@@ -0,0 +1,55 @@
+// 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 System;
+using Xunit;
+
+namespace FakeTests
+{
+    public class GivenThatIWantSomeFakeTests
+    {
+#if NET451
+        [Fact]
+        public void NET451_succeeds()
+        {
+            Assert.True(true);
+        }
+
+        [Fact(Skip="Skipped for NET451")]
+        public void SkippedTest()
+        {
+
+        }
+#else
+        [Fact]
+        public void NETCOREAPP_succeeds()
+        {
+            Assert.True(true);
+        }
+
+        [Fact(Skip="Skipped for NETCOREAPP1.0")]
+        public void SkippedTest()
+        {
+
+        }
+#endif
+
+        [Fact]
+        public void Common_succeeds()
+        {
+            Assert.True(true);
+        }
+
+        [Fact]
+        public void Fails_IfEnvironmentVariableIsSet()
+        {
+            var shouldFail = string.IsNullOrEmpty(Environment.GetEnvironmentVariable("DOTNET_TEST_SHOULD_FAIL"));
+
+#if NET451
+            Assert.True(shouldFail, "Failing in NET451");
+#else
+            Assert.True(shouldFail, "Failing in NETCOREAPP1.0");
+#endif
+        }
+    }
+}