tests: add test for warning-free builds of core under `no_global_oo…

…m_handling`

`tests/run-make/alloc-no-oom-handling` tests that `alloc` under
`no_global_oom_handling` builds and is warning-free.

Do the same for `core` to prevent issues such as [1].

Link: rust-lang#110649 [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

tests/run-make/core-no-oom-handling/Makefile

@@ -0,0 +1,6 @@
+include ../tools.mk
+
+FAKEROOT=$(TMPDIR)/fakeroot
+
+all:
+	$(RUSTC) --edition=2021 -Dwarnings --crate-type=rlib ../../../library/core/src/lib.rs --sysroot=$(FAKEROOT) --cfg no_global_oom_handling