Add test for multi-line attribute handling in doctests

Author: GuillaumeGomez

src/test/rustdoc-ui/doc-comment-multi-line-cfg-attr.rs

@@ -0,0 +1,12 @@
+// compile-flags:--test
+// normalize-stdout-test: "src/test/rustdoc-ui" -> "$$DIR"
+// normalize-stdout-test "finished in \d+\.\d+s" -> "finished in $$TIME"
+// check-pass
+
+/// ```
+/// # #![cfg_attr(not(dox), deny(missing_abi,
+/// # non_ascii_idents))]
+///
+/// pub struct Bar;
+/// ```
+pub struct Bar;

src/test/rustdoc-ui/doc-comment-multi-line-cfg-attr.stdout

@@ -0,0 +1,6 @@
+
+running 1 test
+test $DIR/doc-comment-multi-line-cfg-attr.rs - Bar (line 6) ... ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in $TIME
+