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/string.rs

@@ -11,3 +11,9 @@ impl Display for NixString {
         f.write_str(self.0.as_str())
     }
 }
+
+impl From<&str> for NixString {
+    fn from(s: &str) -> Self {
+        NixString(s.to_string())
+    }
+}