[patch] pg_test_timing does not prompt illegal option

Hi all,

pg_test_timing accepts the following command-line options:
-d duration
--duration=duration

Specifies the test duration, in seconds. Longer durations give slightly better accuracy, and are more likely to discover problems with the system clock moving backwards. The default test duration is 3 seconds.
-V
--version

Print the pg_test_timing version and exit.
-?
--help

Show help about pg_test_timing command line arguments, and exit.

[https://www.postgresql.org/docs/11/pgtesttiming.html]

However, when I use the following command, no error prompt. the command can run.
pg_test_timing --

I think "--" is a illegal option, errors should be prompted.

Here is a patch for prompt illegal option.

Best Regards!