Use $callers, not $ancestors for inline check

We care about Tasks for inline testing checks, not normal supervision
tree ancestry. The $callers entry is the appropriate mechanism to find
the [trail of calling processes][trail]:

[trail]: https://hexdocs.pm/elixir/1.16.2/Task.html#module-ancestor-and-caller-tracking

lib/oban/config.ex

@@ -154,7 +154,7 @@ defmodule Oban.Config do
   def get_engine(%__MODULE__{engine: engine, testing: :disabled}), do: engine
 
   def get_engine(%__MODULE__{engine: engine, testing: testing}) do
-    pids = [self() | Process.get(:"$ancestors", [])]
+    pids = [self() | Process.get(:"$callers", [])]
 
     if Enum.any?(pids, &inline_testing?(&1, testing)) do
       Oban.Engines.Inline
@@ -163,14 +163,12 @@ defmodule Oban.Config do
     end
   end
 
-  defp inline_testing?(pid, default) when is_pid(pid) do
+  defp inline_testing?(pid, default) do
     {:dictionary, dictionary} = Process.info(pid, :dictionary)
 
     Keyword.get(dictionary, :oban_testing, default) == :inline
   end
 
-  defp inline_testing?(_pid, _default), do: false
-
   @doc false
   @spec node_name(%{optional(binary()) => binary()}) :: binary()
   def node_name(env \\ System.get_env()) do

test/oban/testing_test.exs

@@ -405,8 +405,6 @@ defmodule Oban.TestingTest do
 
       Testing.with_testing_mode(:inline, fn ->
         fun = fn ->
-          Process.put(:"$ancestors", [Some.Endpoint | Process.get(:"$ancestors")])
-
           Oban.insert!(name, Worker.new(%{ref: 1, action: "OK"}))
 
           assert_received {:ok, 1}