Introduce module to verify all Oban.insert clauses

The recent addition of a @SPEC for `Oban.insert/2` broke dialyzer in
some situations. To prevent this regression in the future we now include
a compiled module that exercises all `Oban.insert` variations.

Closes oban-bg#762

lib/oban.ex

@@ -241,8 +241,6 @@ defmodule Oban do
   def config(name \\ __MODULE__), do: Registry.config(name)
 
   @doc false
-  @spec insert(Job.changeset(), Keyword.t()) ::
-          {:ok, Job.t()} | {:error, Job.changeset() | term()}
   def insert(%Changeset{} = changeset, opts) do
     insert(__MODULE__, changeset, opts)
   end

test/support/exercise.ex

@@ -0,0 +1,22 @@
+defmodule Oban.Test.Exercise do
+  @moduledoc false
+
+  alias Ecto.Multi
+  alias Oban.Job
+
+  def check_install do
+    changeset = changeset()
+
+    {:ok, _} = Oban.insert(changeset)
+    {:ok, _} = Oban.insert(Oban, changeset)
+    {:ok, _} = Oban.insert(changeset, timeout: 500)
+    {:ok, _} = Oban.insert(Oban, changeset, timeout: 500)
+
+    %Multi{} = Oban.insert(Multi.new(), :job, changeset)
+    %Multi{} = Oban.insert(Multi.new(), :job, changeset, timeout: 500)
+    %Multi{} = Oban.insert(Oban, Multi.new(), :job, changeset)
+    %Multi{} = Oban.insert(Oban, Multi.new(), :job, changeset, timeout: 500)
+  end
+
+  defp changeset, do: Job.new(%{}, worker: "FakeWorker")
+end