test(tvix/eval): test very simple late-bound identifier access

This is the simplest kind of thunk that can be created (and so far the
only one the compiler knows how to create), in which an identifier
inside a `let` encounters a value that is bound *after* it is
initialised.

Change-Id: I6ea4408a3baef1e7d5137365d70804283f2dbf8e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6354
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/tests/tvix_tests/eval-okay-late-binding.exp

@@ -0,0 +1 @@
+42

tvix/eval/src/tests/tvix_tests/eval-okay-late-binding.nix

@@ -0,0 +1,4 @@
+let
+  a = b;
+  b = 42;
+in a