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/dynamic_lib.rs

@@ -117,7 +117,7 @@ impl DynamicLibrary {
 }
 
 #[cfg(all(test, not(target_os = "ios")))]
-mod test {
+mod tests {
     use super::*;
     use prelude::v1::*;
     use libc;