Fix should_panic_without_expect pedantic clippy lint

    warning: #[should_panic] attribute without a reason
       --> tests/test.rs:789:1
        |
    789 | #[should_panic]
        | ^^^^^^^^^^^^^^^ help: consider specifying the expected panic: `#[should_panic(expected = /* panic message */)]`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#should_panic_without_expect
        = note: `-W clippy::should-panic-without-expect` implied by `-W clippy::pedantic`
        = help: to override `-W clippy::pedantic` add `#[allow(clippy::should_panic_without_expect)]`

Author: dtolnay

tests/test.rs

@@ -786,7 +786,7 @@ fn test_invalidate_current_thread_spans() {
 
 #[cfg(span_locations)]
 #[test]
-#[should_panic]
+#[should_panic(expected = "Invalid span with no related FileInfo!")]
 fn test_use_span_after_invalidation() {
     let span = create_span();