Add integration test for escript

[wojtekmach]

This is especially going to be useful to catch bugs like these (I'll fix them later)

 defmodule Foo do @type t() :: String.t() end

fails with:

 Unexpectedly received message {:mix_shell, :run, ["warning: documentation references \"String.t()\" but it is undefined or private\n tmp/ExDoc.CLITest/test-run-from-CLI/lib/foo.ex:6: t:Foo.t/0\n\n"]} (which matched _) code: refute_received _ 

and this is because we don't have docs chunks for Elixir modules when running from escript.

---

However, both on Elixir 1.12 and master, I'm getting the following console output and I'm not sure where it is coming from.

~/src/ex_doc[wm-escript-test]% mix test .......................................................................................................................................................................................... Finished in 8.6 seconds (3.0s async, 5.5s sync) 186 tests, 0 failures Randomized with seed 125690 17:50:15.788 [error] Task #PID<0.362.0> started from #PID<0.93.0> terminating ** (UndefinedFunctionError) undefined function #Function<0.11923361/1 in :elixir_compiler_1>(%{excluded: 0, failures: 0, skipped: 0, total: 186}) (elixir 1.12.1) lib/enum.ex:930: Enum."-each/2-lists^foreach/1-0-"/2 (ex_unit 1.12.1) lib/ex_unit/runner.ex:64: ExUnit.Runner.run_with_trap/2 (ex_unit 1.12.1) lib/ex_unit/runner.ex:31: ExUnit.Runner.run/2 (elixir 1.12.1) lib/task/supervised.ex:90: Task.Supervised.invoke_mfa/2 (elixir 1.12.1) lib/task/supervised.ex:35: Task.Supervised.reply/5 (stdlib 3.15.1) proc_lib.erl:226: :proc_lib.init_p_do_apply/3 Function: #Function<0.95028733/0 in ExUnit.async_run/0> Args: [] ** (EXIT from #PID<0.93.0>) an exception was raised: ** (UndefinedFunctionError) undefined function #Function<0.11923361/1 in :elixir_compiler_1>(%{excluded: 0, failures: 0, skipped: 0, total: 186}) (elixir 1.12.1) lib/enum.ex:930: Enum."-each/2-lists^foreach/1-0-"/2 (ex_unit 1.12.1) lib/ex_unit/runner.ex:64: ExUnit.Runner.run_with_trap/2 (ex_unit 1.12.1) lib/ex_unit/runner.ex:31: ExUnit.Runner.run/2 (elixir 1.12.1) lib/task/supervised.ex:90: Task.Supervised.invoke_mfa/2 (elixir 1.12.1) lib/task/supervised.ex:35: Task.Supervised.reply/5 (stdlib 3.15.1) proc_lib.erl:226: :proc_lib.init_p_do_apply/3