refactor(tvix/eval): use new public API in test code

This removes internal uses of the previous crate::eval module, which
is being removed.

Change-Id: I5fb3c53460a9c5381853d0258f9ed074ab23c630
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7543
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/tests/nix_oracle.rs

@@ -40,8 +40,10 @@ fn nix_eval(expr: &str) -> String {
 #[track_caller]
 fn compare_eval(expr: &str) {
     let nix_result = nix_eval(expr);
-    let tvix_result = tvix_eval::interpret(expr, None, Default::default())
-        .unwrap()
+    let tvix_result = tvix_eval::Evaluation::new(expr, None)
+        .evaluate()
+        .value
+        .expect("tvix evaluation should succeed")
         .to_string();
 
     assert_eq!(nix_result.trim(), tvix_result);