Use refute_timeout configured for all refute_ assertions

What changed?
============

All refutation test helpers in `Bamboo.Test` should be using the
`refute_timeout` configuration when testing on shared mode.

```elixir
config :bamboo, :refute_timeout, 10
```

The `refute_email_delivered_with/1` macro was not using the
configuration, but instead it has 100ms hardcoded. This was a bug, since
the original implementation should have used it.

Known potential issue
----------------------

Since the previous waiting time was 100ms, and the `refute_timeout/0`
function returns 0ms when not using shared mode, then it's possible some
people could experience false positives in their tests (they think the
email isn't received, so test passes. But 10ms later it arrives in the
mailbox. Before, they were waiting 100ms, so they would have caught the
error.)

lib/bamboo/test.ex

@@ -235,16 +235,28 @@ defmodule Bamboo.Test do
   @doc """
   Check that no email was sent with the given parameters
 
-  Similarly to `assert_email_delivered_with`, the assertion waits up to 100ms before
-  failing. Note that you need to send the email again if you want to make other
-  assertions after this, as this will receive the `{:delivered_email, email}` message.
+  Similar to `assert_email_delivered_with/1`, but it checks that an email with
+  those parameters wasn't sent.
+
+  If `Bamboo.Test` is used with shared mode, you must also configure a timeout
+  in your test config.
+
+      # Set this in your config, typically in config/test.exs
+      config :bamboo, :refute_timeout, 10
+
+  The value you set is up to you. Lower values may result in faster tests, but
+  your tests may incorrectly pass if an email is delivered *after* the timeout.
+
+  Note that this assertion helper will grab the email out of the process
+  mailbox. So if you want to make other assertions about the same email after
+  this assertion, you need to send the email again.
 
   ## Examples
 
       Bamboo.Email.new_email(subject: "something") |> MyApp.Mailer.deliver
       refute_email_delivered_with(subject: "something else") # Will pass
 
-      email = Bamboo.Email.new_email(subject: "something") |> MyApp.Mailer.deliver
+      Bamboo.Email.new_email(subject: "something") |> MyApp.Mailer.deliver
       refute_email_delivered_with(subject: ~r/some/) # Will fail
   """
   defmacro refute_email_delivered_with(email_params) do
@@ -255,7 +267,7 @@ defmodule Bamboo.Test do
         receive do
           {:delivered_email, email} -> Map.from_struct(email)
         after
-          100 -> []
+          Bamboo.Test.refute_timeout() -> []
         end
 
       if is_nil(received_email_params) do
@@ -435,7 +447,8 @@ defmodule Bamboo.Test do
     """)
   end
 
-  defp refute_timeout do
+  @doc false
+  def refute_timeout do
     if using_shared_mode?() do
       Application.get_env(:bamboo, :refute_timeout) ||
         raise """