use threads;
use Time::HiRes qw[ time ];
use Test::Whatever;

...

## Pick a time in the future
my $go = time() + 0.1;
async {
    ## Make sure client 2 goes off last (and so should fail)
    $go += 0.005;
    sleep 0 while  time() < $go;
    nok( doXML_RPCrequest( 'some request' ) == SUCCESS );
    sleep 0;
}->detach;

sleep 0 while  time() < $go;
ok( doXML_RPCrequest( 'some request' ) == SUCCESS );

... rest of tests.