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/libstd/io/stdio.rs

@@ -418,7 +418,7 @@ pub fn _print(args: fmt::Arguments) {
 }
 
 #[cfg(test)]
-mod test {
+mod tests {
     use thread;
     use super::*;