test(tvix/eval): test throw in __toString with toJSON

Change-Id: Ia4a9a04c7e157b6add94dc8901ffab35486fe344
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12731
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-json-propagate-catchable.exp

@@ -1 +1 @@
-[ false false false false ]
+[ false false false false false ]

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-json-propagate-catchable.nix

@@ -10,5 +10,8 @@ map (e: (builtins.tryEval (builtins.toJSON e)).success) [
     x = 32;
     y = builtins.throw "second argument";
   }
+  {
+    __toString = _: builtins.throw "__toString a";
+  }
   # FIXME(raitobezarius): we would like to test coercions, i.e. `toFile` and `derivation` containing throwables.
 ]