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/libsyntax/codemap.rs

@@ -949,7 +949,7 @@ pub struct MalformedCodemapPositions {
 //
 
 #[cfg(test)]
-mod test {
+mod tests {
     use super::*;
     use std::rc::Rc;