tokio: gate some panicking tests with #[cfg(panic = "unwind")] (#6110)

tokio/tests/task_blocking.rs

@@ -114,6 +114,7 @@ fn can_enter_current_thread_rt_from_within_block_in_place() {
 }
 
 #[test]
+#[cfg(panic = "unwind")]
 fn useful_panic_message_when_dropping_rt_in_rt() {
     use std::panic::{catch_unwind, AssertUnwindSafe};