test: fix DownloaderTest to retry tempdir cleanup

Author: spikecurtis

Tests.Vpn.Service/DownloaderTest.cs

@@ -231,9 +231,28 @@ public void Setup()
     }
 
     [TearDown]
-    public void TearDown()
+    public async Task TearDown()
     {
-        Directory.Delete(_tempDir, true);
+        const int maxAttempts = 10;
+        for (var attempt = 1; attempt <= maxAttempts; attempt++)
+        {
+            try
+            {
+                Directory.Delete(_tempDir, true);
+                return;
+            }
+            catch (IOException e)
+            {
+                await TestContext.Out.WriteLineAsync(
+                    $"failed to delete temp dir (attempt {attempt} of {maxAttempts}): {e.Message}");
+                if (attempt == maxAttempts)
+                {
+                    throw;
+                }
+
+                await Task.Delay(100);
+            }
+        }
     }
 
     private string _tempDir;