Ignore irrefutable_let_patterns lint in test suite

    warning: irrefutable `if let` pattern
       --> tests/test_ensure.rs:656:30
        |
    656 |     let test = || Ok(ensure!(if let P::<u8> {} = p { 0 } else { 1 } == 1));
        |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: `#[warn(irrefutable_let_patterns)]` on by default
        = note: this pattern will always match, so the `if let` is useless
        = help: consider replacing the `if let` with a `let`

    warning: irrefutable `if let` pattern
       --> tests/test_ensure.rs:662:30
        |
    662 |     let test = || Ok(ensure!(if let ::std::marker::PhantomData = p {} != ()));
        |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: this pattern will always match, so the `if let` is useless
        = help: consider replacing the `if let` with a `let`

tests/test_ensure.rs

@@ -13,7 +13,8 @@
     clippy::too_many_lines,
     clippy::unit_arg,
     clippy::while_immutable_condition,
-    clippy::zero_ptr
+    clippy::zero_ptr,
+    irrefutable_let_patterns
 )]
 
 use anyhow::{anyhow, ensure, Chain, Error, Result};