Set default timeout in test helpers to 100 (beam-community#607)

Commit aa8c458 introduced the ability to accept a timeout option in test
helpers. To consolidate logic, it extracted a `get_timeout/1` function,
but it incorrectly set the default value to 1000 ms. This commit
corrects that by setting it to 100 ms.

We also take the opportunity to use the `get_timeout/1` helper from the
`refute_timeout/1` function, thus we consolidate all fetching of the
`:timeout` option to the `get_timeout/1` helper.

lib/bamboo/test.ex

@@ -477,7 +477,7 @@ defmodule Bamboo.Test do
  but may incorrectly pass if an email is delivered *after* the timeout.
  """
  else
- Keyword.get(opts, :timeout, 100)
+ get_timeout(opts)
  end
  end
 
@@ -493,5 +493,5 @@ defmodule Bamboo.Test do
  end
 
  @doc false
- def get_timeout(opts), do: Keyword.get(opts, :timeout, 1000)
+ def get_timeout(opts), do: Keyword.get(opts, :timeout, 100)
 end