ELIXIR:community-garden add new test to Task4

[zulkris]

there are no necessary test for https://exercism.org/tracks/elixir/exercises/community-garden/ which checking the using plot_id param really used in release function

f.e. current code works well (no tests fails)

  def release(pid, _plot_id) do
    %{next_id: next_id} = Agent.get(pid, & &1)
    Agent.update(pid, fn %{next_id: next_id} -> %{plots: [], next_id: next_id} end)
  end

This PR adds new test to check this

[github-actions]

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Automated comment created by PR Commenter 🤖.

[angelikatyborska]

Well done! That was a terrible oversight from our side, thank you for fixing it 🙂