Suppress unused_attributes warning in test suite

This test has multiple #[ignore] attributes in some configurations.

    $ cargo +beta check --test expandtest
    warning: unused attribute
     --> test_suite/tests/expandtest.rs:2:29
      |
    2 | #[cfg_attr(not(expandtest), ignore)]
      |                             ^^^^^^
      |
      = note: `#[warn(unused_attributes)]` on by default

test_suite/tests/expandtest.rs

@@ -1,6 +1,7 @@
 #[cfg_attr(target_os = "emscripten", ignore)]
 #[cfg_attr(not(expandtest), ignore)]
 #[rustversion::attr(not(nightly), ignore)]
+#[allow(unused_attributes)]
 #[test]
 fn expandtest() {
     macrotest::expand("tests/expand/*.rs");