fix(tvix/eval): proptests require arbitrary feature

`cargo test --no-default-features` fails, if we don't conditionalize
this on the `arbitrary` feature too.

Change-Id: I81a277810119fed0cfc37c942c422f731aa14b2e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11726
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>

tvix/eval/src/value/string.rs

@@ -883,7 +883,7 @@ impl Display for NixString {
     }
 }
 
-#[cfg(test)]
+#[cfg(all(test, feature = "arbitrary"))]
 mod tests {
     use test_strategy::proptest;