Hopefully fix flapping semaphore test

There was a race condition on the $released variable. $t6 released the
semaphore and only then set the $released variable to 1. It's possible that
$t5 ackquired the semaphore and checked the $released variable between this
release and set. So the test would fail because $released was still 0.
First set the variable and then release the semaphore to fix the race.

t/concurrency/03-semaphore.t

@@ -60,8 +60,8 @@ my class Semaphore is repr('Semaphore') { }
     }, 0);
     my $t6 := nqp::newthread({
         nqp::sleep(3.0);
-        nqp::semrelease($s);
         $released := 1;
+        nqp::semrelease($s);
     }, 0);
 
     # First, exhaust semaphore capacity