check_procs: Add delay after forking in test

Forking raises a race condition, where the parent might run the
test before the child has had time to fork. If that happens,
an error similar to this is produced:
 Failed test 'Output correct'
 at ./t/check_procs.t line 32.
                 'PROCS OK: 0 processes with args 'sleep 7' | processes=0;;;0;'
   doesn't match '/^PROCS OK: 1 process?/'

Sleeping a bit should avoid the problem. It might be enough to
sleep less than a second, but perl's built-in sleep function only
supports integer seconds.

In our build environment, the build failed 3 of 4 times before
this patch. After the patch it failed 0 of 7 times.

Signed-off-by: Mikael Falkvidd <mfalkvidd@op5.com>