Allow string interpolation to work with no packages.

tests/data_size.rs

@@ -175,7 +175,7 @@ fn test_max_array_size() -> Result<(), Box<EvalAltResult>> {
     assert_eq!(
         engine.eval::<INT>(
             "
-                let x = [1,2,3];
+                let x = [1,2];
                 len([x, x, x])
             "
         )?,

tests/string.rs

@@ -332,7 +332,8 @@ fn test_string_split() -> Result<(), Box<EvalAltResult>> {
 
 #[test]
 fn test_string_interpolated() -> Result<(), Box<EvalAltResult>> {
-    let engine = Engine::new();
+    // Make sure strings interpolation works even under raw
+    let engine = Engine::new_raw();
 
     assert_eq!(engine.eval::<String>("`${}`")?, "");