Add test

src/tests/Loader/ExternalAssemblyProbe/ExternalAssemblyProbe.cs

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+// 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 Xunit;
+
+public unsafe class ExternalAssemblyProbe
+{
+    [Fact]
+    public static void ExternalAppAssemblies()
+    {
+        // In order to get to this point, the runtime must have been able to find the app assemblies
+        // Check that the TPA is indeed empty - that is, the runtime is not relying on that property.
+        string tpa = AppContext.GetData("TRUSTED_PLATFORM_ASSEMBLIES") as string;
+        Assert.True(string.IsNullOrEmpty(tpa), "TRUSTED_PLATFORM_ASSEMBLIES should be empty");
+    }
+}

src/tests/Loader/ExternalAssemblyProbe/ExternalAssemblyProbe.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <!-- Needed for CLRTestEnvironmentVariable -->
+    <RequiresProcessIsolation>true</RequiresProcessIsolation>
+    <NativeAotIncompatible>true</NativeAotIncompatible>
+    <!-- External assembly probe via host-runtime contract is not implemented.
+         The test uses probing to start at all, so it needs to be disabled in the project, not via an attribute -->
+    <CLRTestTargetUnsupported Condition="'$(RuntimeFlavor)' != 'coreclr'">true</CLRTestTargetUnsupported>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="ExternalAssemblyProbe.cs" />
+
+    <CLRTestEnvironmentVariable Include="APP_ASSEMBLIES" Value="EXTERNAL" />
+  </ItemGroup>
+</Project>