test(tvix/eval): impl Arbitrary for Value

Impl Arbitrary for Value (and NixAttrs and NixList) in the same way we
did for NixString. Value currently only generates non-"internal"
values (no thunks, AttrNotFound, etc.) and can't generate
functions (builtins or closures), because those'd require full
generation of tvix bytecode, which is a bit more work than I'd like to
do now - there's a `todo!` left in the code for a place where we could
allow opting-in to internal values and functions later.

Change-Id: I07a59e2b1d89cfaa912d4ecebd642caf4ddb040a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6627
Autosubmit: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/value/list.rs

@@ -20,6 +20,25 @@ impl Display for NixList {
     }
 }
 
+#[cfg(feature = "arbitrary")]
+mod arbitrary {
+    use proptest::{
+        prelude::{any_with, Arbitrary},
+        strategy::{BoxedStrategy, Strategy},
+    };
+
+    use super::*;
+
+    impl Arbitrary for NixList {
+        type Parameters = <Vec<Value> as Arbitrary>::Parameters;
+        type Strategy = BoxedStrategy<Self>;
+
+        fn arbitrary_with(args: Self::Parameters) -> Self::Strategy {
+            any_with::<Vec<Value>>(args).prop_map(Self).boxed()
+        }
+    }
+}
+
 impl NixList {
     pub fn concat(&self, other: &Self) -> Self {
         let mut lhs = self.clone();