[gdb/testsuite] Fix gdb.python/py-format-string.exp with python 3.13

On fedora rawhide, with python 3.13, I run into:
...
(gdb) python print (gdb.parse_and_eval ('a_point_t').format_string (invalid=True))^M
Python Exception <class 'TypeError'>: \
  this function got an unexpected keyword argument 'invalid'^M
Error occurred in Python: \
  this function got an unexpected keyword argument 'invalid'^M
(gdb) FAIL: $exp: format_string: lang_c: test_all_common: test_invalid_args: \
  a_point_t with option invalid=True
...

A passing version with an older python version looks like:
...
(gdb) python print (gdb.parse_and_eval ('a_point_t').format_string (invalid=True))^M
Python Exception <class 'TypeError'>: \
  'invalid' is an invalid keyword argument for this function^M
Error occurred in Python: \
  'invalid' is an invalid keyword argument for this function^M
(gdb) PASS: $exp: format_string: lang_c: test_all_common: test_invalid_args: \
  a_point_t with option invalid=True
...

Fix this by accepting the updated error message.

Tested on aarch64-linux.

PR testsuite/31912
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31912