feat(tvix/eval): support builtins implemented in Nix itself

This makes it possible to inject builtins into the builtin set that
are written in Nix code, and which at runtime are represented by a
thunk that will compile them the first time they are used.

Change-Id: Ia632367328f66fb2f26cb64ae464f8f3dc9c6d30
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7891
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/eval/src/tests/mod.rs

@@ -2,6 +2,10 @@ use builtin_macros::builtins;
 use pretty_assertions::assert_eq;
 use test_generator::test_resources;
 
+/// Module for one-off tests which do not follow the rest of the
+/// test layout.
+mod one_offs;
+
 #[builtins]
 mod mock_builtins {
     //! Builtins which are required by language tests, but should not