Accept trailing comma in test of impl Debug for PackageId

The standard library is planning to begin emitting trailing commas in
multiline Debug representations to align with the dominant style in
modern Rust code.

      PackageId {
          name: "foo",
          version: "1.0.0",
    -     source: "registry `https://github.com/rust-lang/crates.io-index`"
    +     source: "registry `https://github.com/rust-lang/crates.io-index`",
      }

For now, change this tests to accept both with and without trailing
comma. Once the trailing comma change reaches the stable channel we will
be able to remove one of the cases.

src/cargo/core/package_id.rs

@@ -224,15 +224,32 @@ mod tests {
         let pkg_id = PackageId::new("foo", "1.0.0", SourceId::for_registry(&loc).unwrap()).unwrap();
         assert_eq!(r#"PackageId { name: "foo", version: "1.0.0", source: "registry `https://github.com/rust-lang/crates.io-index`" }"#, format!("{:?}", pkg_id));
 
-        let pretty = r#"
+        let expected = r#"
+PackageId {
+    name: "foo",
+    version: "1.0.0",
+    source: "registry `https://github.com/rust-lang/crates.io-index`",
+}
+"#
+        .trim();
+
+        // Can be removed once trailing commas in Debug have reached the stable
+        // channel.
+        let expected_without_trailing_comma = r#"
 PackageId {
     name: "foo",
     version: "1.0.0",
     source: "registry `https://github.com/rust-lang/crates.io-index`"
 }
 "#
         .trim();
-        assert_eq!(pretty, format!("{:#?}", pkg_id));
+
+        let actual = format!("{:#?}", pkg_id);
+        if actual.ends_with(",\n}") {
+            assert_eq!(actual, expected);
+        } else {
+            assert_eq!(actual, expected_without_trailing_comma);
+        }
     }
 
     #[test]