test(acceptance): give the B256 case a copy folder of its own

Every case of a run shares one root, and the pdfs copy case before it
had already made "copies" there, so the B256 case failed on its mkdir
with EEXIST before it checked anything.

scripts/fuse-acceptance.py

@@ -787,7 +787,9 @@ def test_server_copy_never_copies_an_old_version(ctx: Context) -> None:
     root = ctx.root
     path = root / "being-written.txt"
     write_durable(path, b"first version")
-    dest = root / "copies"
+    # Its own folder: every case of a run shares the root, and "pdfs copy"
+    # already made "copies" there.
+    dest = root / "b256-copies"
     dest.mkdir()
     daemon.wait_for_queue()