Ignore deref_addrof clippy lint in test

    warning: immediately dereferencing a reference
       --> tests/test_ensure.rs:168:40
        |
    168 |     let test = || Ok(ensure!(&mut x == *&&mut &2));
        |                                        ^^^^^^^^^ help: try: `&mut &2`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#deref_addrof
        = note: `#[warn(clippy::deref_addrof)]` on by default

tests/test_ensure.rs

@@ -1,6 +1,7 @@
 #![allow(
     clippy::bool_to_int_with_if,
     clippy::char_lit_as_u8,
+    clippy::deref_addrof,
     clippy::diverging_sub_expression,
     clippy::extra_unused_type_parameters,
     clippy::if_same_then_else,