test(nix/readTree): test marking behavior of readTree

This ensures in a simple example that __readTree and __readTreeChildren
are populated correctly.

Change-Id: I69a46b2ddde0d1f9bf0dff1c4780f033ac8fc27a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3655
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

nix/readTree/tests/test-marker/directory-marked/nested/default.nix

@@ -0,0 +1,3 @@
+{ ... }:
+
+{}