Auto merge of #24783 - jooert:unittestguidelines, r=alexcrichton

Changes the style guidelines regarding unit tests to recommend using a sub-module named "tests" instead of "test" for unit tests as "test" might clash with imports of libtest (see #23870, #24030 and http://users.rust-lang.org/t/guidelines-naming-of-unit-test-module/1078 for previous discussions).

r? @alexcrichton

src/librustc/session/config.rs

@@ -1111,7 +1111,7 @@ impl fmt::Display for CrateType {
 }
 
 #[cfg(test)]
-mod test {
+mod tests {
 
     use session::config::{build_configuration, optgroups, build_session_options};
     use session::build_session;