Fix flaky mount volume tests: refactor into separate test methods with typed Win32 exception handling

src/libraries/Common/tests/System/IO/ReparsePointUtilities.cs

@@ -11,6 +11,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.ComponentModel;
 using System.Diagnostics;
 using System.IO;
 #if !NETFRAMEWORK
@@ -251,7 +252,7 @@ public static void Unmount(string mountPoint)
 
         bool r = DeleteVolumeMountPoint(mountPoint);
         if (!r)
-            throw new Exception(string.Format("Win32 error: {0}", Marshal.GetLastWin32Error()));
+            throw new Win32Exception(Marshal.GetLastWin32Error());
     }
 
     private static ProcessStartInfo CreateProcessStartInfo(string fileName, params string[] arguments)

src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/Delete.Windows.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.ComponentModel;
 using System.Text;
 using Xunit;
 using Microsoft.DotNet.XUnitExtensions;
@@ -82,7 +83,8 @@ public void Delete_VolumeMountPoint()
             {
                 if (Directory.Exists(mountPoint))
                 {
-                    MountHelper.Unmount(mountPoint);
+                    try { MountHelper.Unmount(mountPoint); }
+                    catch (Win32Exception ex) when (ex.NativeErrorCode is 4390 or 3) { }
                     Directory.Delete(mountPoint);
                 }
             }