tests/kernel: kernel.queue.poll: fails if MAX_THREAD_BYTES > 2

The kernel.queue.poll test fails if CONFIG_MAX_THREAD_BYTES is larger
than 2, complaining about no memory for semaphore object. Turns out
the memory pool is not large enough. So make it a bit larger.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>