Try to programmatically pick reserved names that aren't configured instead of hoping they aren't

Author: grynspan

Tests/TestingTests/AttachmentTests.swift

@@ -100,7 +100,13 @@ struct AttachmentTests {
 
 #if os(Windows)
   static let maximumNameCount = Int(_MAX_FNAME)
-  static let reservedNames = ["COM9", "LPT9"]
+  static let reservedNames: [String] = {
+    // Return the list of COM ports that are NOT configured (and so will fail
+    // to open for writing.)
+    (0...9).lazy
+      .flatMap { ["COM\($0)", "LPT\($0)"] }
+      .filter { !PathFileExistsA($0) }
+  }()
 #else
   static let maximumNameCount = Int(NAME_MAX)
   static let reservedNames: [String] = []