Load module during Test

I’m writting tests for a simple module which do HTTP requests.
As recommanded in Mocks and explicit contracts « Plataformatec Blog I use an environnement variable instead of mocking:

defmodule FaLogentriesParserEx do
  @logentries_api Application.get_env(:fa_logentries_parser_ex, :logentries_api)
end

Of course, for dev and prod environment, the module to load in under lib/fa_logentries_parser_ex but since I don’t want test stuff under lib/ I created the “mock” module under test/fa_logentries_parser_ex/logentries_api_fake.exs

My problem: mix test can’t find that module:

  1) test handle_pipeline_status (FaLogentriesParserExTest)
     test/fa_logentries_parser_ex_test.exs:9
     ** (UndefinedFunctionError) function FaLogentriesParserExTest.LogEntriesApiFake.fetch/2 is undefined (module FaLogentriesParserExTest.LogEntriesApiFake is not available)
     stacktrace:
       FaLogentriesParserExTest.LogEntriesApiFake.fetch(1501576495019, 1501576555019)
       (fa_logentries_parser_ex) lib/fa_logentries_parser_ex.ex:10: FaLogentriesParserEx.fetch/1
       test/fa_logentries_parser_ex_test.exs:89: (test)

Could you help me to make that module loadable or advice a better way to test?