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/util/parser_testing.rs

@@ -142,7 +142,7 @@ pub fn is_whitespace(c: char) -> bool {
 }
 
 #[cfg(test)]
-mod test {
+mod tests {
     use super::*;
 
     #[test] fn eqmodws() {