get rid of test-level imports on `tests/*.rs`

[alejandro-vaz]

recently, #495 modularized integration testing, and we have a few files under tests/

some of the tests on those files though, have use statements inside their functions bodies

this use to be necessary because we used to have all the tests for all feature flags in the same file, but that's no longer the case

so simply move all those imports into a top-level item

example:

use smallvec::SmallVec;

#[test]
fn test_serde() {
    use serde_test::{assert_tokens, Token};
    let mut small_vec: SmallVec<i32, 2> = SmallVec::new();
    // ...

should change to

use smallvec::SmallVec;
use serde_test::{assert_tokens, Token};

#[test]
fn test_serde() {
    let mut small_vec: SmallVec<i32, 2> = SmallVec::new();
    // ...