Add feature(offset_of_nested) to core tests

Author: GKFX

library/core/tests/lib.rs

@@ -60,6 +60,7 @@
 #![feature(noop_waker)]
 #![feature(numfmt)]
 #![feature(num_midpoint)]
+#![cfg_attr(not(bootstrap), feature(offset_of_nested))]
 #![feature(isqrt)]
 #![feature(step_trait)]
 #![feature(str_internals)]