Add a test for the shortness format in rustdoc

src/test/rustdoc-ui/issue-81662-shortness.rs

@@ -0,0 +1,12 @@
+// compile-flags:--test --error-format=short
+// normalize-stdout-test: "src/test/rustdoc-ui" -> "$$DIR"
+// normalize-stdout-test "finished in \d+\.\d+s" -> "finished in $$TIME"
+// failure-status: 101
+
+/// ```rust
+/// foo();
+/// ```
+//~^^ ERROR cannot find function `foo` in this scope
+fn foo() {
+    println!("Hello, world!");
+}

src/test/rustdoc-ui/issue-81662-shortness.stdout

@@ -0,0 +1,16 @@
+
+running 1 test
+test $DIR/issue-81662-shortness.rs - foo (line 6) ... FAILED
+
+failures:
+
+---- $DIR/issue-81662-shortness.rs - foo (line 6) stdout ----
+$DIR/issue-81662-shortness.rs:7:1: error[E0425]: cannot find function `foo` in this scope
+error: aborting due to previous error
+Couldn't compile the test.
+
+failures:
+    $DIR/issue-81662-shortness.rs - foo (line 6)
+
+test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in $TIME
+