Add and use keyfile_to_key_id() helper function

When trying to extract the key ID from a key file name, some test code incorrectly attempts to strip all leading zeros. This breaks tests when keys with ID 0 are generated. Add a new helper shell function, keyfile_to_key_id(), which properly handles keys with ID 0 and use it in test code whenever a key ID needs to be extracted from a key file name.