Reduce test iterations

pthreads seems to be having trouble with such quick generation of threads. Will look into what may be happening here… but the number of iterations isn't terribly important for the modified test.

test/Worker/AbstractPoolTest.php

@@ -134,7 +134,7 @@ public function testBusyPool() {
     public function testCleanGarbageCollection() {
         // See https://github.com/amphp/parallel-functions/issues/5
         Loop::run(function () {
-            for ($i = 0; $i < 15; $i++) {
+            for ($i = 0; $i < 3; $i++) {
                 $pool = $this->createPool(32);
 
                 $values = \range(1, 50);