move completion item tests closer to the code

this is the reason why we need marks: the tests were spread across two
files, because I've forgotten that there were tests already

crates/ra_hir/src/lib.rs

@@ -8,8 +8,6 @@
 pub mod db;
 #[cfg(test)]
 mod mock;
-#[cfg(test)]
-mod marks;
 mod query_definitions;
 mod path;
 pub mod source_binder;
@@ -29,6 +27,9 @@ mod generics;
 mod code_model_api;
 mod code_model_impl;
 
+#[cfg(test)]
+mod marks;
+
 use crate::{
     db::HirDatabase,
     name::{AsName, KnownName},