Restore UI test involving missing Display impl

This test existed prior to 04b91d7646.

Since 791a98eb93, we can rely on rust-src being installed.

tests/ui/missing-display.rs

@@ -0,0 +1,9 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum MyError {
+    First,
+    Second,
+}
+
+fn main() {}

tests/ui/missing-display.stderr

@@ -0,0 +1,8 @@
+error[E0277]: `MyError` doesn't implement `std::fmt::Display`
+  --> tests/ui/missing-display.rs:4:1
+   |
+4  | pub enum MyError {
+   | ^^^^^^^^^^^^^^^^ `MyError` cannot be formatted with the default formatter
+   |
+   = help: the trait `std::fmt::Display` is not implemented for `MyError`
+   = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead