Change name of unit test sub-module to "tests".

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.

src/libstd/thread/mod.rs

@@ -722,7 +722,7 @@ fn _assert_sync_and_send() {
 ////////////////////////////////////////////////////////////////////////////////
 
 #[cfg(test)]
-mod test {
+mod tests {
     use prelude::v1::*;
 
     use any::Any;