Add test for rustdoc cfg(test) feature

src/test/rustdoc-ui/cfg-test.rs

+// compile-pass
+// compile-flags:--test
+// normalize-stdout-test: "src/test/rustdoc-ui" -> "$$DIR"
+
+/// this doctest will be ignored:
+///
+/// ```
+/// assert!(false);
+/// ```
+#[cfg(not(test))]
+pub struct Foo;
+
+/// this doctest will be tested:
+///
+/// ```
+/// assert!(true);
+/// ```
+#[cfg(test)]
+pub struct Foo;

src/test/rustdoc-ui/cfg-test.stdout

+
+running 1 test
+test $DIR/cfg-test.rs - Foo (line 15) ... ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
+