Auto merge of #2393 - WiSaGaN:bugfix/tests-mod-naming, r=alexcrichton

In the default `lib.rs` provided by `cargo-new`, the "test" module is named "test" while Rust convention is using "tests" as the name of the module: https://doc.rust-lang.org/book/testing.html#the-tests-module
The plural form also conforms with the style used in special directories at project root such as "examples" and "tests".
This pull request conforms to the existing style.