Add unittest

Add unittest
[   28s] + ./spin
[   28s]
[   28s] running 32 tests
[   28s] test mutex::spin::tests::smoke ... ok
[   28s] test mutex::spin::tests::test_into_inner ... ok
[   28s] test mutex::spin::tests::test_into_inner_drop ... ok
[   28s] test mutex::spin::tests::lots_and_lots ... ok
[   28s] test mutex::spin::tests::test_mutex_force_lock ... ok
[   28s] test mutex::spin::tests::test_mutex_unsized ... ok
[   28s] test mutex::spin::tests::try_lock ... ok
[   28s] test once::tests::drop_occurs_and_skip_uninit_drop ... ok
[   28s] test once::tests::get ... ok
[   28s] test mutex::spin::tests::test_mutex_arc_access_in_unwind ... ok
[   28s] test mutex::spin::tests::test_mutex_arc_nested ... ok
[   28s] test once::tests::init_constant ... ok
[   28s] test once::tests::panic ... ok
[   28s] test once::tests::poll ... ok
[   28s] test once::tests::smoke_once_value ... ok
[   28s] test once::tests::smoke_once ... ok
[   28s] test barrier::tests::test_barrier ... ok
[   28s] test rwlock::tests::smoke ... ok
[   28s] test rwlock::tests::test_force_read_decrement ... ok
[   28s] test rwlock::tests::test_force_write_unlock ... ok
[   28s] test rwlock::tests::test_into_inner ... ok
[   28s] test rwlock::tests::test_into_inner_drop ... ok
[   28s] test once::tests::wait ... ok
[   28s] test rwlock::tests::test_rwlock_try_write ... ok
[   28s] test rwlock::tests::test_rwlock_unsized ... ok
[   28s] test rwlock::tests::test_rw_try_read ... ok
[   28s] test rwlock::tests::test_upgrade_downgrade ... ok
[   28s] test once::tests::stampede_once ... ok
[   28s] test rwlock::tests::test_rw_access_in_unwind ... ok
[   28s] test rwlock::tests::test_rw_arc ... ok
[   28s] test once::tests::call_once_test ... ok
[   31s] test once::tests::get_no_wait ... ok
[   31s]
[   31s] test result: ok. 32 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 3.06s