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/ast.rs

@@ -1869,7 +1869,7 @@ pub struct MacroDef {
 }
 
 #[cfg(test)]
-mod test {
+mod tests {
     use serialize;
     use super::*;