[CERTTF-413] fix: include chmod method in the tarfile patch (#368)

This allows for directories to be extracted. The filtered `mode`
for directories is `None` and the unpatched `chmod` is not
forgiving of that.

agent/testflinger_agent/tarfile_patch.py

@@ -258,6 +258,15 @@ def extractall(
             except ExtractError as e:
                 self._handle_nonfatal_error(e)
 
+    def chmod(self, tarinfo, targetpath):
+        """Set file permissions of targetpath according to tarinfo."""
+        if tarinfo.mode is None:
+            return
+        try:
+            os.chmod(targetpath, tarinfo.mode)
+        except OSError:
+            raise ExtractError("could not change mode")
+
 
 # make sure `open` is available when this module is imported and it
 # returns a `TarFilePatched` object instead of a `TarFile` one