Add new rustdoc-ui test to ensuire that rustdoc feature isn't passed down to doctests

src/test/rustdoc-ui/doc-test-rustdoc-feature.rs

@@ -0,0 +1,14 @@
+// build-pass
+// compile-flags:--test
+// normalize-stdout-test: "src/test/rustdoc-ui" -> "$$DIR"
+
+#![feature(doc_cfg)]
+
+// Make sure `cfg(rustdoc)` is set when finding doctests but not inside the doctests.
+
+/// ```
+/// #![feature(doc_cfg)]
+/// assert!(!cfg!(rustdoc));
+/// ```
+#[cfg(rustdoc)]
+pub struct Foo;

src/test/rustdoc-ui/doc-test-rustdoc-feature.stdout

@@ -0,0 +1,6 @@
+
+running 1 test
+test $DIR/doc-test-rustdoc-feature.rs - Foo (line 9) ... ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
+