Configure retry backoff at compile time

The option makes testing with_retry much faster, but it isn't documented
for the time being.

config/config.exs

@@ -4,6 +4,8 @@ config :elixir, :time_zone_database, Tz.TimeZoneDatabase
 
 config :logger, level: :warning
 
+config :oban, Oban.Backoff, retry_mult: 1
+
 config :oban, Oban.Test.Repo,
   migration_lock: false,
   name: Oban.Test.Repo,

lib/oban/backoff.ex

@@ -3,6 +3,8 @@ defmodule Oban.Backoff do
 
   @type jitter_mode :: :inc | :dec | :both
 
+  @retry_mult Application.compile_env(:oban, [Oban.Backoff, :retry_mult], 100)
+
   @doc """
   Calculate an exponential backoff in seconds for a given attempt.
 
@@ -115,7 +117,7 @@ defmodule Oban.Backoff do
   defp retry_or_raise(fun, retries, attempt, kind, reason, stacktrace) do
     if retries == :infinity or attempt < retries do
       attempt
-      |> exponential(mult: 100)
+      |> exponential(mult: @retry_mult)
       |> jitter()
       |> Process.sleep()