In `PrettyTestRunner`, when a test case fails, prioritize using a folder without a numbered suffix for the output directory.

ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs

@@ -877,6 +877,8 @@ namespace ICSharpCode.Decompiler.Tests
 			string baseTempName = testName + Tester.GetSuffix(cscOptions);
 			string tempDir = Path.Combine(Path.GetTempPath(), "ICSharpCode.Decompiler.Tests", baseTempName);
 			if (Directory.Exists(tempDir))
+			{
+				void PrepareNewDirName()
 				{
 					int idx = 2;
 					string newDir;
@@ -891,6 +893,23 @@ namespace ICSharpCode.Decompiler.Tests
 						idx++;
 					}
 				}
+				const bool preferOverwrite = true;
+				if (preferOverwrite)
+				{
+					try
+					{
+						Tester.RepeatOnIOError(() => Directory.Delete(tempDir, true));
+					}
+					catch
+					{
+						PrepareNewDirName();
+					}
+				}
+				else
+				{
+					PrepareNewDirName();
+				}
+			}
 			Directory.CreateDirectory(tempDir);
 
 			string originalIl = Path.Combine(tempDir, Path.GetFileNameWithoutExtension(exeFile) + ".original.il");