Activate the feature in the libcore tests too

src/libcore/tests/lib.rs

@@ -19,6 +19,7 @@
 #![feature(flt2dec)]
 #![feature(fmt_internals)]
 #![feature(hashmap_internals)]
+#![feature(impl_header_lifetime_elision)]
 #![feature(pattern)]
 #![feature(range_is_empty)]
 #![feature(raw)]