add test demonstrating problems with formatting

this test looks right, and if you run with `UPDATE_EXPECT=1` it is not
changed, but if you run without it, you get an error

Author: nikomatsakis

tests/judgment-error-reporting/fallible.rs

@@ -0,0 +1,49 @@
+use anyhow::Context;
+use formality_core::{judgment_fn, term, test, test_util::ResultTestExt, Fallible};
+
+#[term]
+enum Ty {
+    Class { name: ClassName },
+}
+
+formality_core::id!(ClassName);
+
+judgment_fn! {
+    fn sub(
+        a: Ty,
+        b: Ty,
+    ) => () {
+        debug(a, b)
+
+        (
+            (if name_a == name_b)
+            ---------------------- ("same class")
+            (sub(Ty::Class { name: name_a }, Ty::Class { name: name_b }) => ())
+        )
+    }
+}
+
+fn check_sub(a: Ty, b: Ty) -> Fallible<()> {
+    Ok(sub(a, b)
+        .check_proven()
+        .with_context(|| format!("check_sub"))?)
+}
+
+#[test]
+fn test() {
+    let foo = Ty::Class {
+        name: ClassName::new("Foo"),
+    };
+    let bar = Ty::Class {
+        name: ClassName::new("Bar"),
+    };
+    // Demonstrates a multi-line error rendered by anyhow.
+    check_sub(foo, bar).assert_err(expect_test::expect![[r#"
+        check_sub
+
+        Caused by:
+            judgment `sub { a: class(Foo), b: class(Bar) }` failed at the following rule(s):
+              the rule "same class" failed at step #0 (src/file.rs:LL:CC) because
+                condition evaluted to false: `name_a == name_b`
+    "#]]);
+}

tests/judgment-error-reporting/main.rs

@@ -20,6 +20,7 @@ formality_core::declare_language! {
 use jer::FormalityLang;
 
 mod cyclic_judgment;
+mod fallible;
 mod grammar;
 
 fn main() -> Fallible<()> {