library: add cfg test as known cfg

This is needed because since rust-lang/cargo#15007 `test` is no longer a
built-in well-known cfg when `[lib] test = false` is set.

Author: weihanglo

library/core/Cargo.toml

@@ -31,6 +31,7 @@ check-cfg = [
     'cfg(no_fp_fmt_parse)',
     'cfg(stdarch_intel_sde)',
     'cfg(target_arch, values("xtensa"))',
+    'cfg(test)',
     # core use #[path] imports to portable-simd `core_simd` crate
     # and to stdarch `core_arch` crate which messes-up with Cargo list
     # of declared features, we therefor expect any feature cfg