test(sandbox): cover directory copy actions across sandbox modes

test/blackbox-tests/test-cases/sandbox/copy-dir-deps.t

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+A copy action on a directory dependency should produce regular files in every
+sandbox mode. This case has no overlapping dependencies.
+
+  $ make_dune_project 3.25
+  $ cat > dune <<'EOF'
+  > (rule
+  >  (target (dir tree))
+  >  (action (system "mkdir tree && echo contents > tree/file")))
+  > (rule
+  >  (target (dir copied))
+  >  (deps tree (sandbox always))
+  >  (action (no-infer (copy tree copied))))
+  > EOF
+
+First check that the directory dependency contains a regular file.
+
+  $ dune build tree/file
+  $ dune_cmd stat kind _build/default/tree/file
+  regular file
+
+Symlink mode: the copy action follows the directory link and copies its files.
+
+  $ dune build copied --sandbox symlink
+  $ cat _build/default/copied/file
+  contents
+  $ dune_cmd stat kind _build/default/copied/file
+  regular file
+
+Copy mode also produces a regular file with the expected contents.
+
+  $ dune build copied --sandbox copy
+  $ cat _build/default/copied/file
+  contents
+  $ dune_cmd stat kind _build/default/copied/file
+  regular file
+
+Hardlink mode currently fails while populating the directory dependency,
+before the copy action runs.
+
+  $ dune build copied --sandbox hardlink 2>&1 | censor
+  Error:
+  link(_build/.sandbox/$DIGEST/default/tree): Operation not permitted
+  -> required by _build/default/copied
+  [1]