Update logic of mixed mode of dotnet test (#47407)

Author: mariam-abdulla

src/Cli/dotnet/commands/dotnet-test/LocalizableStrings.resx

@@ -317,7 +317,9 @@ For MSTest before 2.2.4, the timeout is used for all testcases.</value>
     <comment>{0} - test message state</comment>
   </data>
   <data name="CmdUnsupportedVSTestTestApplicationsDescription" xml:space="preserve">
-    <value>Test project '{0}' is using VSTest. When opting-in via dotnet.config, all the test projects are expected to be using Microsoft.Testing.Platform. 
+    <value>When opting-in via dotnet.config, all test projects are expected to be using Microsoft.Testing.Platform.
+The following test projects are using VSTest:
+{0}
 See https://aka.ms/dotnet-test/mtp for more information.</value>
   </data>
   <data name="Aborted" xml:space="preserve">

src/Cli/dotnet/commands/dotnet-test/MSBuildHandler.cs

@@ -1,9 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System;
 using System.Collections.Concurrent;
-using System.IO;
+using System.Diagnostics;
 using Microsoft.DotNet.Tools.Common;
 using Microsoft.DotNet.Tools.Test;
 using Microsoft.Testing.Platform.OutputDevice;
@@ -91,21 +90,28 @@ private int RunBuild(string filePath, bool isSolution, BuildOptions buildOptions
 
         private void InitializeTestApplications(IEnumerable<TestModule> modules)
         {
+            // If one test app has IsTestingPlatformApplication set to false (VSTest and not MTP), then we will not run any of the test apps
+            IEnumerable<TestModule> vsTestTestProjects = modules.Where(module => !module.IsTestingPlatformApplication);
+
+            if (vsTestTestProjects.Any())
+            {
+                _areTestingPlatformApplications = false;
+
+                _output.WriteMessage(
+                    string.Format(
+                        LocalizableStrings.CmdUnsupportedVSTestTestApplicationsDescription,
+                        string.Join(Environment.NewLine, vsTestTestProjects.Select(module => Path.GetFileName(module.ProjectFullPath)))),
+                    new SystemConsoleColor { ConsoleColor = ConsoleColor.Red });
+
+                return;
+            }
+
             foreach (TestModule module in modules)
             {
                 if (!module.IsTestProject && !module.IsTestingPlatformApplication)
                 {
-                    // This should never happen. We should only ever create Module if it's a test project.
-                    throw new InvalidOperationException();
-                }
-
-                if (!module.IsTestingPlatformApplication)
-                {
-                    // If one test app has IsTestingPlatformApplication set to false (VSTest and not MTP), then we will not run any of the test apps
-                    // Note that we still continue the loop here so that we print all projects that are VSTest and not MTP.
-                    _areTestingPlatformApplications = false;
-                    _output.WriteMessage(string.Format(LocalizableStrings.CmdUnsupportedVSTestTestApplicationsDescription, Path.GetFileName(module.ProjectFullPath)), new SystemConsoleColor { ConsoleColor = ConsoleColor.Red });
-                    continue;
+                    // This should never happen. We should only ever create TestModule if it's a test project.
+                    throw new UnreachableException($"This program location is thought to be unreachable. Class='{nameof(MSBuildHandler)}' Method='{nameof(InitializeTestApplications)}'");
                 }
 
                 var testApp = new TestApplication(module, _args);