tests/task: workaround only for FreeBSD < 13

I added this workaround in:
    2018-10-05 tests/task: workaround for FreeBSD non-POSIX behaviour
    a526143

This was fixed in FreeBSD 13.0.0 and up:
freebsd/freebsd-src@77da4a9

tests/task/test-task.cpp

@@ -119,7 +119,7 @@ void TestTask::sleep_filenotfound()
     RUN_SCRIPT("sleep-1-filenotfound.sh", false);
     // We got a "finished" signal, with sh exit code 127 ("command not found").
     QVERIFY(sig_fin.count() == 1);
-#if defined(Q_OS_FREEBSD)
+#if defined(__FreeBSD__) && (__FreeBSD__ < 13)
     // Bug in FreeBSD
     QVERIFY(sig_fin.takeFirst().at(1).toInt() == 2);
 #else