Add test for race condition in community gardens

If the `register` function uses `get` and `update` instead of
`get_and_update` concurrent operations results in non-unique ids.

exercises/concept/community-garden/test/community_garden_test.exs

@@ -102,4 +102,24 @@ defmodule CommunityGardenTest do
     assert {:ok, pid} = CommunityGarden.start()
     assert {:not_found, "plot is unregistered"} = CommunityGarden.get_registration(pid, 1)
   end
+
+  @tag task_id: 6
+  test "register multiple plots concurrently" do
+    {:ok, pid} = CommunityGarden.start()
+
+    # Spawn 25 processes that register a plot concurrently
+    for _ <- 1..25 do
+      Task.async(fn -> CommunityGarden.register(pid, "user") end)
+    end
+    |> Enum.map(&Task.await/1)
+    |> Enum.map(& &1.plot_id)
+
+    # Get the list of registered plot IDs
+    plot_ids =
+      CommunityGarden.list_registrations(pid)
+      |> Enum.map(& &1.plot_id)
+
+    # Assert that each plot ID is unique
+    assert Enum.uniq(plot_ids) == plot_ids
+  end
 end