Ignore incompatible_msrv clippy false positives in test

rust-lang/rust-clippy#12257

    warning: current MSRV (Minimum Supported Rust Version) is `1.31.0` but this item is stable since `1.57.0`
     --> tests/fst/mod.rs:5:44
      |
    5 |     fst::Set::from(fst::raw::Fst::new(data.as_slice()).unwrap())
      |                                            ^^^^^^^^^^
      |
      = 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 `1.31.0` but this item is stable since `1.57.0`
      --> tests/fst/mod.rs:10:44
       |
    10 |     fst::Set::from(fst::raw::Fst::new(data.as_slice()).unwrap())
       |                                            ^^^^^^^^^^
       |
       = 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 `1.31.0` but this item is stable since `1.32.0`
      --> tests/compare.rs:46:48
       |
    46 |             xid_start_fst.contains((ch as u32).to_be_bytes()),
       |                                                ^^^^^^^^^^^^^
       |
       = 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 `1.31.0` but this item is stable since `1.32.0`
      --> tests/compare.rs:51:51
       |
    51 |             xid_continue_fst.contains((ch as u32).to_be_bytes()),
       |                                                   ^^^^^^^^^^^^^
       |
       = 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 `1.31.0` but this item is stable since `1.32.0`
      --> benches/xid.rs:88:44
       |
    88 |                 let ch_bytes = (ch as u32).to_be_bytes();
       |                                            ^^^^^^^^^^^^^
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incompatible_msrv

benches/xid.rs

@@ -12,7 +12,10 @@
 // readme is computed by subtracting this baseline from the other bench
 // functions' time, then dividing by one million (ms -> ns).
 
-#![allow(clippy::needless_pass_by_value)]
+#![allow(
+    clippy::incompatible_msrv, // https://github.com/rust-lang/rust-clippy/issues/12257
+    clippy::needless_pass_by_value,
+)]
 
 #[path = "../tests/fst/mod.rs"]
 mod fst;

tests/compare.rs

@@ -1,3 +1,7 @@
+#![allow(
+    clippy::incompatible_msrv, // https://github.com/rust-lang/rust-clippy/issues/12257
+)]
+
 mod fst;
 mod roaring;
 mod trie;