tests: use minicore more

minicore makes it much easier to add new language items to all of the
existing `no_core` tests.

tests/codegen/function-return.rs

@@ -1,6 +1,7 @@
 // Test that the `fn_ret_thunk_extern` function attribute is (not) emitted when
 // the `-Zfunction-return={keep,thunk-extern}` flag is (not) set.
 
+//@ add-core-stubs
 //@ revisions: unset keep thunk-extern keep-thunk-extern thunk-extern-keep
 //@ needs-llvm-components: x86
 //@ compile-flags: --target x86_64-unknown-linux-gnu
@@ -13,8 +14,8 @@
 #![feature(no_core, lang_items)]
 #![no_core]
 
-#[lang = "sized"]
-trait Sized {}
+extern crate minicore;
+use minicore::*;
 
 #[no_mangle]
 pub fn foo() {