Move some rustdoc-ui tests to subdirectories

tests/rustdoc-ui/doctest/cfg-test.rs

@@ -0,0 +1,31 @@
+// check-pass
+// compile-flags:--test --test-args --test-threads=1
+// normalize-stdout-test: "tests/rustdoc-ui/doctest" -> "$$DIR"
+// normalize-stdout-test "finished in \d+\.\d+s" -> "finished in $$TIME"
+
+// Crates like core have doctests gated on `cfg(not(test))` so we need to make
+// sure `cfg(test)` is not active when running `rustdoc --test`.
+
+/// this doctest will be ignored:
+///
+/// ```
+/// assert!(false);
+/// ```
+#[cfg(test)]
+pub struct Foo;
+
+/// this doctest will be tested:
+///
+/// ```
+/// assert!(true);
+/// ```
+#[cfg(not(test))]
+pub struct Foo;
+
+/// this doctest will be tested, but will not appear in documentation:
+///
+/// ```
+/// assert!(true)
+/// ```
+#[cfg(doctest)]
+pub struct Bar;