Add unittest

Add unittest
[   20s] + ./smawk
[   20s]
[   20s] running 19 tests
[   20s] test monge::tests::is_monge_handles_overflow ... ok
[   20s] test monge::tests::monge_constant_cols ... ok
[   20s] test monge::tests::monge_constant_rows ... ok
[   20s] test monge::tests::monge_lower_left ... ok
[   20s] test monge::tests::monge_upper_right ... ok
[   20s] test tests::online_1x1 ... ok
[   20s] test tests::online_2x2 ... ok
[   20s] test tests::online_4x4 ... ok
[   20s] test tests::online_3x3 ... ok
[   20s] test tests::online_5x5 ... ok
[   20s] test tests::online_works_with_partial_ord ... ok
[   20s] test tests::smawk_2x2 ... ok
[   20s] test tests::smawk_1x2 ... ok
[   20s] test tests::smawk_1x1 ... ok
[   20s] test tests::smawk_3x3 ... ok
[   20s] test tests::smawk_2x1 ... ok
[   20s] test tests::smawk_4x4 ... ok
[   20s] test tests::smawk_5x5 ... ok
[   20s] test tests::smawk_works_with_partial_ord ... ok
[   20s]
[   20s] test result: ok. 19 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.03s