Put str tests in a nested tests module

src/lexer/str.rs

@@ -62,8 +62,12 @@ impl CharAt for str {
     }
 }
 
-#[test]
-fn index_before_is_well_behaved_for_ascii() {
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn index_before_is_well_behaved_for_ascii() {
         let s = "abc";
 
         // Sanity
@@ -79,10 +83,10 @@ fn index_before_is_well_behaved_for_ascii() {
             assert_eq!(last_char.len(), 1);
             assert_eq!(last_char.chars().nth(0), Some('c'));
         }
-}
+    }
 
-#[test]
-fn index_after_is_well_behaved_for_ascii() {
+    #[test]
+    fn index_after_is_well_behaved_for_ascii() {
         let s = "abc";
 
         // Sanity
@@ -95,4 +99,5 @@ fn index_after_is_well_behaved_for_ascii() {
             assert_eq!(idx, s.len());
             assert!(s.is_char_boundary(idx));
         }
+    }
 }