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_ide_api/src/lib.rs

@@ -26,6 +26,9 @@ mod syntax_highlighting;
 mod parent_module;
 mod rename;
 
+#[cfg(test)]
+mod marks;
+
 use std::{fmt, sync::Arc};
 
 use ra_syntax::{SourceFile, TreeArc, TextRange, TextUnit};