feat(nix/readTree): add tests of the traversal logic in README

This should closely match the documented behaviour. It might still be
missing some edge cases of course.

Change-Id: I5c75fa045d5f3be8cf5eab787a02644500c14522
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2466
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

nix/readTree/tests/test-tree-traversal/no-skip-subtree/default.nix

@@ -0,0 +1,5 @@
+{ ... }:
+
+{
+  but = "the default.nix is still read";
+}