Clean up test side effects in DAP message tests

We were depending on the test run to succeed in order to create and
overwrite the file; however, this was very confusing when debugging a
single test while it was failing.

Author: andyleejordan

test/PowerShellEditorServices.Test.E2E/DebugAdapterProtocolMessageTests.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Microsoft Corporation.
+// Copyright (c) Microsoft Corporation.
 // Licensed under the MIT License.
 
 using System;
@@ -127,7 +127,13 @@ private string GenerateScriptFromLoggingStatements(params string[] logStatements
                 throw new ArgumentNullException(nameof(logStatements), "Expected at least one argument.");
             }
 
-            // Have script create/overwrite file first with `>`.
+            // Clean up side effects from other test runs.
+            if (File.Exists(s_testOutputPath))
+            {
+                File.Delete(s_testOutputPath);
+            }
+
+            // Have script create file first with `>` (but don't rely on overwriting).
             StringBuilder builder = new StringBuilder().Append('\'').Append(logStatements[0]).Append("' > '").Append(s_testOutputPath).AppendLine("'");
             for (int i = 1; i < logStatements.Length; i++)
             {