test(tvix/value): add simple attrset construction tests

These do not yet test nested attribute sets; we need to add some more
inspection primitives first.

Change-Id: Icfc99bf17c73ebefc0d882a84f0ca73ec688a54d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6110
Reviewed-by: eta <tvl@eta.st>
Tested-by: BuildkiteCI

tvix/eval/src/value/attrs.rs

@@ -14,6 +14,9 @@ use crate::errors::{Error, EvalResult};
 use super::string::NixString;
 use super::Value;
 
+#[cfg(test)]
+mod tests;
+
 #[derive(Clone, Debug)]
 pub enum NixAttrs {
     Empty,