test: Add failing test pulling empty directory from cache. (#11116)

Signed-off-by: Rudi Grinberg <[email protected]>

test/blackbox-tests/test-cases/dune-cache/empty-dir.t

@@ -0,0 +1,34 @@
+Check the cache restores empty directories
+
+  $ export DUNE_CACHE=enabled
+  $ export DUNE_CACHE_ROOT=$PWD/dune-cache
+  $ cat >dune-project <<EOF
+  > (lang dune 3.10)
+  > (using directory-targets 0.1)
+  > EOF
+
+  $ cat >dune <<EOF
+  > (rule
+  >  (target (dir output))
+  >  (action
+  >   (progn
+  >    (run mkdir output)
+  >    (run mkdir output/child)
+  >    (run touch output/file))))
+  > EOF
+
+Build an empty directory.
+
+  $ dune build output
+  $ find _build/default/output | sort
+  _build/default/output
+  _build/default/output/child
+  _build/default/output/file
+
+Restore it from cache.
+
+  $ rm -rf _build
+  $ dune build output
+  $ find _build/default/output | sort
+  _build/default/output
+  _build/default/output/file