import only required function

Based on hexdocs: https://hexdocs.pm/elixir/alias-require-and-import.html\#import it is recommended to only import the function you are using. This change proposes to just import `render_to_string` in the `error_html_test.exs` file

installer/templates/phx_test/controllers/error_html_test.exs

@@ -2,7 +2,7 @@ defmodule <%= @web_namespace %>.ErrorHTMLTest do
   use <%= @web_namespace %>.ConnCase, async: true
 
   # Bring render_to_string/4 for testing custom views
-  import Phoenix.Template
+  import Phoenix.Template, only [render_to_string: 4]
 
   test "renders 404.html" do
     assert render_to_string(<%= @web_namespace %>.ErrorHTML, "404", "html", []) == "Not Found"