tests: update test cases for newer syntax items; drop old artifacts

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I8640148e8e7597924f9c776750c856266a6a6964

tests/ancient_let.nix

@@ -0,0 +1,8 @@
+# Test ancient let syntax: let { bindings; body = expr; }
+# This is equivalent to: let bindings in expr, but has been deprecated
+# in newer Nix versions.
+let {
+  x = 10;
+  y = 20;
+  body = x + y;
+}