Range checks of pg_test_fsync --secs-per-test and pg_test_timing --duration

Hi all,

As $subject says, pg_test_fsync and pg_test_timing don't really check
the range of option values specified.  It is possible for example to
make pg_test_fsync run an infinite amount of time, and pg_test_timing
does not handle overflows with --duration at all.

These are far from being critical issues, but let's fix them at least
on HEAD.  So, please see the attached, where I have also added some
basic TAP tests for both tools.

Thanks,
--
Michael