Retain time as microseconds for scheduled_at tests

Include microseconds in the `begin` and `until` times used for scheduled_at
tests with a delta. The prior version would truncate, which rounded the
`until` down and broke microsecond level checks.

Closes #1162

lib/oban/testing.ex

@@ -661,10 +661,10 @@ defmodule Oban.Testing do
         time -> {time, 1}
       end
 
-    {:ok, time} = Ecto.Type.cast(:utc_datetime, time)
+    datetime = Ecto.Type.cast!(:utc_datetime_usec, time)
 
-    begin = DateTime.add(time, -delta, :second)
-    until = DateTime.add(time, +delta, :second)
+    begin = DateTime.add(datetime, -delta, :second)
+    until = DateTime.add(datetime, +delta, :second)
 
     where(query, [j], fragment("? BETWEEN ? AND ?", field(j, ^key), ^begin, ^until))
   end

test/oban/testing_test.exs

@@ -272,11 +272,13 @@ defmodule Oban.TestingTest do
     test "checking for jobs with matching timestamps within delta" do
       insert!(%{}, worker: Ping)
       insert!(%{}, worker: Pong, scheduled_at: seconds_from_now(60))
+      insert!(%{}, worker: Pong, scheduled_at: ~U[2000-10-20 05:10:31.057748Z])
 
       assert_enqueued worker: Ping, state: "available", scheduled_at: DateTime.utc_now()
       assert_enqueued worker: Ping, scheduled_at: DateTime.now!("America/Chicago")
       assert_enqueued worker: Pong, scheduled_at: seconds_from_now(60)
       assert_enqueued worker: Pong, scheduled_at: {seconds_from_now(69), delta: 10}
+      assert_enqueued worker: Pong, scheduled_at: ~U[2000-10-20 05:10:30.557748Z]
     end
 
     test "asserting that jobs are now or will eventually be enqueued" do