Ignore incompatible_msrv clippy false positives in test

rust-lang/rust-clippy#12257

    warning: current MSRV (Minimum Supported Rust Version) is `1.56.0` but this item is stable since `1.71.0`
        --> tests/test.rs:2500:25
         |
    2500 |         assert_ne!(rand.hash_one(k1), rand.hash_one(k2));
         |                         ^^^^^^^^^^^^
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incompatible_msrv
         = note: `-W clippy::incompatible-msrv` implied by `-W clippy::all`
         = help: to override `-W clippy::all` add `#[allow(clippy::incompatible_msrv)]`

    warning: current MSRV (Minimum Supported Rust Version) is `1.56.0` but this item is stable since `1.71.0`
        --> tests/test.rs:2500:44
         |
    2500 |         assert_ne!(rand.hash_one(k1), rand.hash_one(k2));
         |                                            ^^^^^^^^^^^^
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incompatible_msrv

    warning: current MSRV (Minimum Supported Rust Version) is `1.56.0` but this item is stable since `1.71.0`
        --> tests/test.rs:2503:25
         |
    2503 |         assert_eq!(rand.hash_one(k1), rand.hash_one(k2));
         |                         ^^^^^^^^^^^^
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incompatible_msrv

    warning: current MSRV (Minimum Supported Rust Version) is `1.56.0` but this item is stable since `1.71.0`
        --> tests/test.rs:2503:44
         |
    2503 |         assert_eq!(rand.hash_one(k1), rand.hash_one(k2));
         |                                            ^^^^^^^^^^^^
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incompatible_msrv

tests/test.rs

@@ -5,6 +5,7 @@
     clippy::derive_partial_eq_without_eq,
     clippy::excessive_precision,
     clippy::float_cmp,
+    clippy::incompatible_msrv, // https://github.com/rust-lang/rust-clippy/issues/12257
     clippy::items_after_statements,
     clippy::let_underscore_untyped,
     clippy::shadow_unrelated,