Hide the quote_*! macros when the feature gate is off

This makes it easier to experiment with improved quasiquoting as an ordinary
plugin library.

The list of quote macros in feature_gate.rs was already out of sync;
this commit also prevents that problem in the future.

src/libsyntax/test.rs

@@ -235,10 +235,7 @@ fn generate_test_harness(sess: &ParseSess,
         sess: sess,
         span_diagnostic: sd,
         ext_cx: ExtCtxt::new(sess, cfg.clone(),
-                             ExpansionConfig {
-                                 deriving_hash_type_parameter: false,
-                                 crate_name: "test".to_string(),
-                             }),
+                             ExpansionConfig::default("test".to_string())),
         path: Vec::new(),
         testfns: Vec::new(),
         reexport_test_harness_main: reexport_test_harness_main,