externals: Add catch2 v3.2.1

Merge commit '6879e5bb1c598a9a517d7bdec8ba1a0bace3ef10' as 'externals/catch'

externals/catch/tools/scripts/checkDuplicateFilenames.py

@@ -0,0 +1,14 @@
+#!/usr/bin/env python3
+
+import os
+import sys
+
+files_set = set()
+
+for root, dir, files in os.walk("src/catch2"):
+    for file in files:
+        if file not in files_set:
+            files_set.add(file)
+        else:
+            print("File %s is duplicate" % file)
+            sys.exit(1)