Move the Cache Control header test to its own file

We can consolidate our cache control header tests here

test/plugs/cache_control_test.exs

@@ -0,0 +1,20 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2018 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.CacheControlTest do
+  use Pleroma.Web.ConnCase
+  alias Plug.Conn
+
+  test "Verify Cache-Control header on static assets", %{conn: conn} do
+    conn = get(conn, "/index.html")
+
+    assert Conn.get_resp_header(conn, "cache-control") == ["public, no-cache"]
+  end
+
+  test "Verify Cache-Control header on the API", %{conn: conn} do
+    conn = get(conn, "/api/v1/instance")
+
+    assert Conn.get_resp_header(conn, "cache-control") == ["max-age=0, private, must-revalidate"]
+  end
+end