Fix flaky changes async test

Try to prevent failures like:
```
  1) test continuous filtered changes (ChangesAsyncTest)
     src/couchdb/test/elixir/test/changes_async_test.exs:193
     Expected truthy, got false
     code: assert Enum.member?(changes_ids, "rusty")
     arguments:

         # 1
         ["bingo"]

         # 2
         "rusty"

     stacktrace:
       src/couchdb/test/elixir/test/changes_async_test.exs:220: (test)

```

test/elixir/test/changes_async_test.exs

@@ -209,16 +209,18 @@ defmodule ChangesAsyncTest do
     :ok = wait_for_headers(req_id.id, 200)
     create_doc(db_name, %{_id: "rusty", bop: "plankton"})
 
-    changes = process_response(req_id.id, &parse_changes_line_chunk/1)
+    retry_until(fn ->
+      changes = process_response(req_id.id, &parse_changes_line_chunk/1)
 
-    changes_ids =
-      changes
-      |> Enum.filter(fn p -> Map.has_key?(p, "id") end)
-      |> Enum.map(fn p -> p["id"] end)
+      changes_ids =
+        changes
+        |> Enum.filter(fn p -> Map.has_key?(p, "id") end)
+        |> Enum.map(fn p -> p["id"] end)
 
-    assert Enum.member?(changes_ids, "bingo")
-    assert Enum.member?(changes_ids, "rusty")
-    assert length(changes_ids) == 2
+      Enum.member?(changes_ids, "bingo") and
+      Enum.member?(changes_ids, "rusty") and
+      length(changes_ids) == 2
+    end)
   end
 
   @tag :with_db