Add test using private reflection

src/tests/Interop/CommandLine/CommandLineTest.cs

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+using System;
+using System.IO;
+using System.Reflection;
+using TestLibrary;
+using Xunit;
+
+namespace CommandLineTest
+{
+    class Program
+    {
+        int Main()
+        {
+            // Currently native command line is only implemented on CoreCLR Windows
+            if (!TestLibrary.Utilities.IsMonoRuntime && !TestLibrary.Utilities.IsNativeAot)
+            {
+                // Clear the command line args set for managed entry point
+                var field = typeof(Environment).GetField("s_commandLineArgs", BindingFlags.Static | BindingFlags.NonPublic);
+                Assert.NotNull(field);
+                field.SetValue(null, null);
+
+                string[] args = Environment.GetCommandLineArgs();
+                if (OperatingSystem.IsWindows())
+                {
+                    // The command line should be "corerun assemblyname.dll" for coreclr test
+                    Assert.Equal(2, args.Length);
+                    Assert.Equal("corerun", Path.GetFileNameWithoutExtension(args[0]));
+                    Assert.Equal(typeof(Program).Assembly.GetName().Name, Path.GetFileNameWithoutExtension(args[1]));
+                }
+            }
+
+            return 100;
+        }
+    }
+}

src/tests/Interop/CommandLine/CommandLineTest.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="CommandLineTest.cs" />
+  </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="$(TestSourceDir)Common/CoreCLRTestLibrary/CoreCLRTestLibrary.csproj" />
+  </ItemGroup>
+</Project>