normalize all test names to not start with `test_`

[alejandro-vaz]

replace all tests in tests/*.rs whose function name starts with test_ and remove that part

currently, some tests have it, some other don't, and a consistent pattern is more important than which one it is

examples of some that don't have test_ on tests/main.rs:909:

#[test]
fn empty_macro() {
    let _v: SmallVec<u8, 1> = smallvec![];
}

#[test]
fn zero_size_items() {
    SmallVec::<(), 0>::new().push(());
}

most other tests do though

so just remove test_ from all