router.ex

  defmodule AppWeb.Router do
  # ...
    scope "/", AppWeb do
      pipe_through :browser
      live "/test", TestLive
    end
  end
  

test_component.ex

defmodule AppWeb.TestComponent do
  use AppWeb, :live_component

  def render(assigns) do
    ~H"""
    <div>
      <.form let={_f} for={:form} action="#" phx-submit="submit">
        <%= submit("submit") %>
      </.form>

      <%= if @redirected do %>
        <p>redirected</p>
      <% end %>
    </div>
    """
  end
end

test_live.ex

defmodule AppWeb.TestLive do
  use AppWeb, :live_view

  @impl true
  def render(assigns) do
    ~H"""
    <.live_component module={AppWeb.TestComponent} id={1} {assigns}/>
    """
  end

  @impl true
  def mount(_params, _session, socket) do
    socket =
      socket
      |> assign_defaults()
      |> assign_new(:redirected, fn -> false end)

    {:ok, socket, layout: false}
  end

  defp assign_defaults(socket) do
    attach_hook(socket, :set_current_url, :handle_params, fn
      _params, url, socket ->
        uri = URI.parse(url)

        if uri.query do
          {:cont, assign(socket, current_url: uri.path <> "?" <> uri.query)}
        else
          {:cont, assign(socket, current_url: uri.path)}
        end
    end)
  end

  @impl true
  def handle_event("submit", _, socket) do
    if socket.assigns.current_url do
      {:noreply, assign(socket, :redirected, true)}
    else
      {:noreply, socket}
    end
  end
end

test_live_test.exs

defmodule AppWeb.TestLiveTest do
  use AppWeb.ConnCase

  import Phoenix.LiveViewTest

  test "renders redirected", %{conn: conn} do
    {:ok, view, html} = live(conn, Routes.live_path(conn, AppWeb.TestLive))
    assert html =~ "submit"
    refute html =~ "redirected"

    view
    |> element("form")
    |> render_submit()

    assert render(view) =~ "redirected"
  end
end