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/ext/expand.rs

@@ -1553,7 +1553,7 @@ impl<'a, 'v> Visitor<'v> for MacroExterminator<'a> {
 
 
 #[cfg(test)]
-mod test {
+mod tests {
     use super::{pattern_bindings, expand_crate};
     use super::{PatIdentFinder, IdentRenamer, PatIdentRenamer, ExpansionConfig};
     use ast;