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.
b9dfb4d6f8