Turn the nonmodrs-mods test into a standard idempotence test

We need to skip children on foo.rs, since the parser will not find bar from that
file, but with that, the test works fine.

src/test/mod.rs

@@ -232,23 +232,6 @@ fn self_tests() {
     );
 }
 
-#[test]
-fn issue_2673_non_modrs_mods() {
-    match idempotent_check(&PathBuf::from("tests/issue-2673-nonmodrs-mods/lib.rs")) {
-        Ok(ref report) if report.has_warnings() => {
-            print!("{}", report);
-            panic!("had warnings");
-        }
-        Ok(_report) => {}
-        Err(err) => {
-            if let IdempotentCheckError::Mismatch(msg) = err {
-                print_mismatches_default_message(msg);
-            }
-            panic!("had errors");
-        }
-    }
-}
-
 #[test]
 fn stdin_formatting_smoke_test() {
     let input = Input::Text("fn main () {}".to_owned());

tests/config/skip_children.toml

@@ -0,0 +1 @@
+skip_children = true

tests/target/issue-2673-nonmodrs-mods/foo.rs

@@ -0,0 +1,4 @@
+// rustfmt-config: skip_children.toml
+mod bar;
+
+mod baz {}

tests/target/issue-2673-nonmodrs-mods/lib.rs

@@ -0,0 +1,6 @@
+#![feature(non_modrs_mods)]
+
+// Test that submodules in non-mod.rs files work. This is just an idempotence
+// test since we just want to verify that rustfmt doesn't fail.
+
+mod foo;