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/parse/token.rs

@@ -746,7 +746,7 @@ pub fn fresh_mark() -> ast::Mrk {
 }
 
 #[cfg(test)]
-mod test {
+mod tests {
     use super::*;
     use ast;
     use ext::mtwt;