Fix test test_hash

[lexoooooo]

Compline fails on my machine (Clippy is on). And I found the test implement for Hash is incorrect.

x.hash() returns a () instead of an u32. So the test is meaningless. It will always succeed.

I write a pitch using a function hash() to compute a hash value :)

error: `assert_eq` of unit values detected. This will always succeed
   --> src/tests.rs:608:9
    |
608 |         assert_eq!(a.hash(&mut hasher), b.hash(&mut hasher));
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unit_cmp
    = note: `#[deny(clippy::unit_cmp)]` on by default

[mbrubeck]

Thanks!