replace feature expression (cfg_panic) in lib and remove expression from tests

Rebase commit

Author: chiw-goog

core/tests/lib.rs

@@ -6,7 +6,7 @@
 #![feature(bool_to_option)]
 #![feature(box_syntax)]
 #![feature(cell_update)]
-#![feature(cfg_panic)]
+#![cfg_attr(bootstrap,feature(cfg_panic))]
 #![cfg_attr(bootstrap, feature(cfg_target_has_atomic))]
 #![feature(const_assume)]
 #![feature(const_black_box)]