Adapt the testsuite baseline to the new Opt_Parse error message

Author: HugoGGuerrier

testsuite/tests/gnatcheck_errors/multiple_lkql_rule_files/test.out

@@ -1,4 +1,4 @@
 testsuite_driver: No output file generated by gnatcheck
-gnatcheck: error: invalid switch: --rule-file
+gnatcheck: error: for option rule-file - switch is present multiple times
 try "gnatcheck --help" for more information.
 >>>program returned status code 2