[Bug] - Add better mocking to tests

[ilyichv]

Describe the bug We want to have better mocking in order to have stronger tests. What we can do is add a support file which contains a map that simulates existing units, after that we should update persistence_fn.ex following functions

def execute(query, :all), do: {:execute, :all}
  def execute(query, :one), do: nil
  def execute(query, :raw), do: {:execute, :raw}
  def execute(query, :count), do: {:execute, :count}
  def execute(query, :pseudo_query), do: {:execute, :pseudo_query}