Ignore char_lit_as_u8 clippy lint in test

    warning: casting a character literal to `u8` truncates
       --> tests/test_ensure.rs:495:30
        |
    495 |     let test = || Ok(ensure!('\0' as u8 > 1));
        |                              ^^^^^^^^^^ help: use a byte literal instead: `b'\0'`
        |
        = note: `char` is four bytes wide, but `u8` is a single byte
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#char_lit_as_u8
        = note: `#[warn(clippy::char_lit_as_u8)]` on by default

    warning: casting a character literal to `u8` truncates
       --> tests/test_ensure.rs:498:30
        |
    498 |     let test = || Ok(ensure!('\0' as ::std::primitive::u8 > 1));
        |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use a byte literal instead: `b'\0'`
        |
        = note: `char` is four bytes wide, but `u8` is a single byte
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#char_lit_as_u8

tests/test_ensure.rs

@@ -1,5 +1,6 @@
 #![allow(
     clippy::bool_to_int_with_if,
+    clippy::char_lit_as_u8,
     clippy::diverging_sub_expression,
     clippy::extra_unused_type_parameters,
     clippy::if_same_then_else,