Put str tests in a nested tests module

src/lexer/str.rs

@@ -62,6 +62,10 @@ impl CharAt for str {
     }
 }
 
+#[cfg(test)]
+mod tests {
+    use super::*;
+
     #[test]
     fn index_before_is_well_behaved_for_ascii() {
         let s = "abc";
@@ -96,3 +100,4 @@ fn index_after_is_well_behaved_for_ascii() {
             assert!(s.is_char_boundary(idx));
         }
     }
+}