Add UI test for --extra-doctests command-line flag

Author: GuillaumeGomez

tests/rustdoc-ui/extract-doctests.rs

@@ -0,0 +1,14 @@
+// Test to ensure that it generates expected output for `--extract-doctests` command-line
+// flag.
+
+//@ compile-flags:-Z unstable-options --extract-doctests
+//@ check-pass
+
+//! ```ignore
+//! let x = 12;
+//! let y = 14;
+//! ```
+//!
+//! ```edition2018,compile_fail
+//! let
+//! ```

tests/rustdoc-ui/extract-doctests.stdout

@@ -0,0 +1 @@
+[{"filename":"tests/rustdoc-ui/extract-doctests.rs","line":7,"langstr":{"original":"ignore","should_panic":false,"no_run":false,"ignore":"All","rust":true,"test_harness":false,"compile_fail":false,"standalone_crate":false,"error_codes":[],"edition":null,"added_classes":[],"unknown":[]},"text":"let x = 12;\nlet y = 14;"},{"filename":"tests/rustdoc-ui/extract-doctests.rs","line":12,"langstr":{"original":"edition2018,compile_fail","should_panic":false,"no_run":true,"ignore":"None","rust":true,"test_harness":false,"compile_fail":true,"standalone_crate":false,"error_codes":[],"edition":"2018","added_classes":[],"unknown":[]},"text":"let"}]