Fix test failure due to quoting change in str's Debug

Beginning with nightly-2021-03-27:

    note: panic did not contain expected string
          panic message: `"\"'a#\" is not a valid Ident"`,
     expected substring: `"\"\\'a#\" is not a valid Ident"`

    failures:
        lifetime_invalid

tests/test.rs

@@ -1,4 +1,5 @@
 use proc_macro2::{Ident, Literal, Punct, Spacing, Span, TokenStream, TokenTree};
+use std::panic;
 use std::str::{self, FromStr};
 
 #[test]
@@ -71,9 +72,24 @@ fn lifetime_number() {
 }
 
 #[test]
-#[should_panic(expected = r#""\'a#" is not a valid Ident"#)]
 fn lifetime_invalid() {
-    Ident::new("'a#", Span::call_site());
+    let result = panic::catch_unwind(|| Ident::new("'a#", Span::call_site()));
+    match result {
+        Err(box_any) => {
+            let message = box_any.downcast_ref::<String>().unwrap();
+            let expected1 = r#""\'a#" is not a valid Ident"#; // 1.31.0 .. 1.53.0
+            let expected2 = r#""'a#" is not a valid Ident"#; // 1.53.0 ..
+            assert!(
+                message == expected1 || message == expected2,
+                "panic message does not match expected string\n\
+                 \x20   panic message: `{:?}`\n\
+                 \x20expected message: `{:?}`",
+                message,
+                expected2,
+            );
+        }
+        Ok(_) => panic!("test did not panic as expected"),
+    }
 }
 
 #[test]