Ensure prerequisites of csc-only tests

test/dotnet.Tests/CommandTests/Run/RunFileTestBase.cs

@@ -140,7 +140,7 @@ private static string PrepareOutOfTreeBaseDirectory()
     /// is created under this directory, and NuGet walks up from the project location to
     /// find config files.
     /// </summary>
-    protected static void CopyNuGetConfigToRunfileDirectory()
+    internal static void CopyNuGetConfigToRunfileDirectory()
     {
         var sourceNuGetConfig = Path.Join(SdkTestContext.Current.TestExecutionDirectory, "NuGet.config");
         var runfileDir = VirtualProjectBuilder.GetTempSubdirectory();

test/dotnet.Tests/CommandTests/Run/RunFileTests_CscOnlyAndApi.cs

@@ -8,10 +8,33 @@
 using Microsoft.DotNet.Cli.Utils;
 using Microsoft.DotNet.FileBasedPrograms;
 using Microsoft.DotNet.ProjectTools;
+using Xunit.Sdk;
 
 namespace Microsoft.DotNet.Cli.Run.Tests;
 
-public sealed class RunFileTests_CscOnlyAndApi(ITestOutputHelper log) : RunFileTestBase(log)
+public sealed class RunFileCscOnlyFixture(IMessageSink sink) : IAsyncLifetime
+{
+    public ValueTask InitializeAsync()
+    {
+        RunFileTestBase.CopyNuGetConfigToRunfileDirectory();
+
+        // Ensure a simple app runs fully with MSBuild before running other csc-only tests
+        // so we have packages like ILLink.Tasks restored and csc-only optimization can kick in.
+        new DotnetCommand(new SharedTestOutputHelper(sink), "run", "-")
+            .WithStandardInput("""
+                Console.WriteLine("Hello");
+                """)
+            .Execute()
+            .Should().Pass()
+            .And.HaveStdOut("Hello");
+
+        return default;
+    }
+
+    public ValueTask DisposeAsync() => default;
+}
+
+public sealed class RunFileTests_CscOnlyAndApi(ITestOutputHelper log) : RunFileTestBase(log), IClassFixture<RunFileCscOnlyFixture>
 {
     [Fact]
     public void UpToDate()