Add test

Author: thiamsantos

test/bypass_test.exs

@@ -584,4 +584,10 @@ defmodule BypassTest do
 
     Mix.Config.persist(bypass: [test_framework: :ex_unit])
   end
+
+  test "Bypass.open/1 raises when cannot start child" do
+    assert_raise RuntimeError, ~r/Failed to start bypass instance/, fn ->
+      Bypass.open(:error)
+    end
+  end
 end