Fix calling 'plan' twice

`plan` is called twice on not Mac such as Linux.
I got following error message on my Linux machine.

````
% prove t/33-specify-kqueue.t
t/33-specify-kqueue.t .. 
1..9
You tried to plan twice at t/33-specify-kqueue.t line 15.
# Looks like your test exited with 255 before it could output anything.
Dubious, test returned 255 (wstat 65280, 0xff00)
Failed 9/9 subtests 

Test Summary Report
-------------------
t/33-specify-kqueue.t (Wstat: 65280 Tests: 0 Failed: 0)
  Non-zero exit status: 255
  Parse errors: Bad plan.  You planned 9 tests but ran 0.
Files=1, Tests=0,  0 wallclock secs ( 0.02 usr  0.01 sys +  0.22 cusr  0.02 csys =  0.27 CPU)
Result: FAIL
````

I fix that `plan` is called only once.
Please review this patch.