Ignore incompatible_msrv clippy false positives in test

rust-lang/rust-clippy#12257

    warning: current MSRV (Minimum Supported Rust Version) is `1.39.0` but this item is stable since `1.64.0`
      --> tests/test_macros.rs:64:39
       |
    64 |         future::ready(anyhow!("...")).await;
       |                                       ^^^^^
       |
       = 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 `1.39.0` but this item is stable since `1.48.0`
      --> tests/test_macros.rs:64:9
       |
    64 |         future::ready(anyhow!("...")).await;
       |         ^^^^^^^^^^^^^
       |
       = 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 `1.39.0` but this item is stable since `1.64.0`
      --> tests/test_macros.rs:72:59
       |
    72 |         future::ready(anyhow!(message(Cell::new("...")))).await;
       |                                                           ^^^^^
       |
       = 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 `1.39.0` but this item is stable since `1.48.0`
      --> tests/test_macros.rs:72:9
       |
    72 |         future::ready(anyhow!(message(Cell::new("...")))).await;
       |         ^^^^^^^^^^^^^
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incompatible_msrv

tests/test_macros.rs

@@ -1,6 +1,7 @@
 #![allow(
     clippy::assertions_on_result_states,
     clippy::eq_op,
+    clippy::incompatible_msrv, // https://github.com/rust-lang/rust-clippy/issues/12257
     clippy::items_after_statements,
     clippy::needless_pass_by_value,
     clippy::shadow_unrelated,