Ignore unnecessary_cast clippy lint in test

    warning: casting integer literal to `u8` is unnecessary
       --> tests/test_ensure.rs:634:30
        |
    634 |     let test = || Ok(ensure!(0 as int!(...) != 0));
        |                              ^^^^^^^^^^^^^^ help: try: `0_u8`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast
        = note: `#[warn(clippy::unnecessary_cast)]` on by default

    warning: casting integer literal to `u8` is unnecessary
       --> tests/test_ensure.rs:637:30
        |
    637 |     let test = || Ok(ensure!(0 as int![...] != 0));
        |                              ^^^^^^^^^^^^^^ help: try: `0_u8`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast

    warning: casting integer literal to `u8` is unnecessary
       --> tests/test_ensure.rs:640:30
        |
    640 |     let test = || Ok(ensure!(0 as int! {...} != 0));
        |                              ^^^^^^^^^^^^^^^ help: try: `0_u8`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast

tests/test_ensure.rs

@@ -19,6 +19,7 @@
     clippy::redundant_pattern_matching,
     clippy::too_many_lines,
     clippy::unit_arg,
+    clippy::unnecessary_cast,
     clippy::while_immutable_condition,
     clippy::zero_ptr,
     irrefutable_let_patterns