Rollup merge of #129592 - saethlin:core-cfg-test, r=tgross35

Remove cfg(test) from library/core

The diff here is very small with the ignore whitespace option.

`core` doesn't/can't have unit tests. All of its tests are just modules under `tests/`, so it has no use for `cfg(test)`, because the entire contents of `library/core/src` are only ever compiled with that cfg off, and the entire contents of `library/core/tests` are only ever compiled with that cfg on.

You can tell this is what's happening because we had `#[cfg(test)]` on a module declaration that has no source file.

I also deleted the extra `mod tests {` layer of nesting; there's no need to mention again in the module path that this is a module of tests. This exposes a name collision between the `u128` module of tests and `core::u128`. Fixed that by using `<u128>::MAX` like is done in the `check!` macro, which is what avoids this name ambiguity for the other types.

library/core/src/error.rs

@@ -1,9 +1,6 @@
 #![doc = include_str!("error.md")]
 #![stable(feature = "error_in_core", since = "1.81.0")]
 
-#[cfg(test)]
-mod tests;
-
 use crate::any::TypeId;
 use crate::fmt::{Debug, Display, Formatter, Result};
 

library/core/tests/lib.rs

@@ -141,7 +141,6 @@ mod intrinsics;
 mod io;
 mod iter;
 mod lazy;
-#[cfg(test)]
 mod macros;
 mod manually_drop;
 mod mem;