Add tests for doc_keyword feature extension

Author: GuillaumeGomez

src/test/rustdoc-ui/invalid-keyword.rs

@@ -0,0 +1,4 @@
+#![feature(doc_keyword)]
+
+#[doc(keyword = "foo df")] //~ ERROR
+mod foo {}

src/test/rustdoc-ui/invalid-keyword.stderr

@@ -0,0 +1,8 @@
+error: `foo df` is not a valid identifier
+  --> $DIR/invalid-keyword.rs:3:17
+   |
+LL | #[doc(keyword = "foo df")]
+   |                 ^^^^^^^^
+
+error: aborting due to previous error
+

src/test/rustdoc/keyword.rs

@@ -14,3 +14,8 @@
 #[doc(keyword = "match")]
 /// this is a test!
 mod foo{}
+
+// @has foo/keyword.foo.html '//section[@id="main"]//div[@class="docblock"]//p' 'hello'
+#[doc(keyword = "foo")]
+/// hello
+mod bar {}