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/librustdoc/html/toc.rs

@@ -198,7 +198,7 @@ impl fmt::Display for Toc {
 }
 
 #[cfg(test)]
-mod test {
+mod tests {
     use super::{TocBuilder, Toc, TocEntry};
 
     #[test]