[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
discoverproblems 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!