Make async test more robust

tests/Composer/Test/Util/ProcessExecutorTest.php

@@ -119,10 +119,13 @@ class ProcessExecutorTest extends TestCase
     {
         $process = new ProcessExecutor($buffer = new BufferIO('', StreamOutput::VERBOSITY_DEBUG));
         $process->enableAsync();
+        $start = microtime(true);
         /** @var Promise $promise */
-        $promise = $process->executeAsync('echo foo');
+        $promise = $process->executeAsync('sleep 2');
         $this->assertEquals(1,  $process->countActiveJobs());
         $promise->cancel();
         $this->assertEquals(0,  $process->countActiveJobs());
+        $end = microtime(true);
+        $this->assertTrue($end - $start < 0.5, 'Canceling took longer than it should, lasted '.($end - $start));
     }
 }