ex_tldr_test: insert Mox and write a possible test

It is a "possible test" because it can not be implemented as it is,
because HTTPoison does not raise the defined exception. But it is
interesting to keep the test for the future implementation of the
necessary behaviour to adress this issue.

test/ex_tldr_test.exs

@@ -1,8 +1,13 @@
 defmodule ExTldrTest do
   use ExUnit.Case
   import ExUnit.CaptureIO
+  alias ClientMock
   doctest ExTldr
 
+  import Mox
+
+  setup :verify_on_exit!
+
   test "look for a linux command" do
     execute_main = fn ->
       ExTldr.main(["linux", "adduser"])
@@ -58,4 +63,12 @@ defmodule ExTldrTest do
 
     assert capture_io(execute_main) =~ "Usage:"
   end
+
+  #test "should return lack of internet" do
+    # assert_raise NoInternetConnectionError,
+    #             fn ->
+    #               ClientMock
+    #               |> expect(:get, fn _ -> {:error, %HTTPoison.Error{reason: :nxdomain}} end)
+    #             end
+  #end
 end