fix(tvix/eval): fix current clippy warnings

It's been a while since the last time, so quite a lot of stuff has
accumulated here.

Change-Id: I0762827c197b30a917ff470fd8ae8f220f6ba247
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7597
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/tests/mod.rs

@@ -49,15 +49,13 @@ fn eval_test(code_path: &str, expect_success: bool) {
                 "{code_path}: test passed unexpectedly!  consider moving it out of notyetpassing"
             );
         }
+    } else if expect_success {
+        panic!("{code_path}: should be able to read test expectation");
     } else {
-        if expect_success {
-            panic!("{code_path}: should be able to read test expectation");
-        } else {
-            panic!(
-                "{code_path}: test should have failed, but succeeded with output {}",
-                result_str
-            );
-        }
+        panic!(
+            "{code_path}: test should have failed, but succeeded with output {}",
+            result_str
+        );
     }
 }