Do not crash when moduledoc and other functions are listed in only, closes elixir-lang#12120

Author: josevalim

lib/ex_unit/lib/ex_unit/doc_test.ex

@@ -264,7 +264,7 @@ defmodule ExUnit.DocTest do
         {tests, fun_arities} -> {[test | tests], [test.fun_arity | fun_arities]}
       end
 
-    case only -- fun_arities do
+    case only -- [:moduledoc | fun_arities] do
       [] ->
         filtered_tests
 

lib/ex_unit/test/ex_unit/doc_test_test.exs

@@ -462,7 +462,10 @@ defmodule ExUnit.DocTestTest do
   # doctest ExUnit.DocTest
 
   doctest ExUnit.DocTestTest.GoodModule, import: true
-  doctest ExUnit.DocTestTest.SomewhatGoodModuleWithOnly, only: [one: 0], import: true
+
+  doctest ExUnit.DocTestTest.SomewhatGoodModuleWithOnly,
+    only: [:moduledoc, one: 0],
+    import: true
 
   doctest ExUnit.DocTestTest.SomewhatGoodModuleWithExcept,
     except: [:moduledoc, two: 0],