add compiletest headers to rustdoc-gui tests

tests/rustdoc-gui/src/extend_css/lib.rs

@@ -1 +1,2 @@
+// compile-flags: --extend-css extra.css
 //! <div class="extend">text in red</div>

tests/rustdoc-gui/src/link_to_definition/lib.rs

@@ -1,3 +1,4 @@
+// compile-flags: -Zunstable-options --generate-link-to-definition
 pub fn sub_fn() {
     barbar();
 }

tests/rustdoc-gui/src/scrape_examples/src/lib.rs

@@ -1,3 +1,4 @@
+// run-flags:-Zrustdoc-scrape-examples
 /// # Examples
 ///
 /// ```