Move `#[cfg(test)]` modules into separate files to save recompiling the `std` crate
Implements an accepted proposal: https://github.com/rust-lang/compiler-team/issues/344
Some notes for reviewers:
* `mod tests` nested in `mod foo` in `mod bar`, I move `foo` to a new file, `tests` is a new file in foo: For example library/std/src/sys/sgx/abi/tls.rs
* `mod test` (not `mod tests`) also is moved.
* `mod benches` are moved.
* `mod tests` is placed before any `use` statements: The topic is discussed in https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Code.20Style.20process
* Some files in cloudabi was changed too. But I notice copyright banners in those files, should we ping cloudabi people?
* I formatted files after moving tests around. I think that may make it easier to review :p .
* Some files don't need `ignore-tidy-filelength` anymore.
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