Tests: Adjust code and annotations for poll test cases.

1. Shorten the long function body for test_poll_wait() to
increase readability, split it into below two method:
    test_poll_wait(),
    check_results().

2. Adjust annotations for below poll test cases:
    test_poll_wait(),
    test_poll_cancel(),
    test_poll_threadstate().

Signed-off-by: YouhuaX Zhu <youhuax.zhu@intel.com>