Bug 1808609 - Ignore non .txt files in test_site_compatibility.py, r=ahochheiden

This ensures we aren't accidentally mistaking e.g, .rej files as valid sites.

Depends on D165998

Differential Revision: https://phabricator.services.mozilla.com/D165999

python/mach/mach/test/test_site_compatibility.py

@@ -21,6 +21,9 @@ def _resolve_command_site_names():
         if not child.is_file():
             continue
 
+        if child.suffix != ".txt":
+            continue
+
         if child.name == "mach.txt":
             continue