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/liballoc/heap.rs

@@ -384,7 +384,7 @@ mod imp {
 }
 
 #[cfg(test)]
-mod test {
+mod tests {
     extern crate test;
     use self::test::Bencher;
     use boxed::Box;