您的消息中缺少某些内容。从您发布的内容中，我们可以了解到每隔@interval毫秒就会发送一条:work消息。您没有告诉我们在发送消息时handle_info/2应该执行什么操作。
一旦定义了它，就可以使用assert_receivedAssert编写一个测试来Assert消息已经收到。

I would test do_smt() by using Mock library and writing a test that makes as assertion like the following:

with_mock(MyModule, [do_stm_else: fn -> :ok]) do
   do_smt()

   assert_called MyModule.do_stm_else()
end

In this way, you have called the function that the task should execute, so you can assume that the task creation is being called.
If you want to let the do_stm_else function communicate with your test (in this scenario it looks a bit overengineered) you should:

1. get the pid of the test by calling self()
2. Pass the pid to the mock function to get it used
3. use assert_receive to verify that the communication has occurred

pid = self()

with_mock(MyModule, [do_stm_else: fn -> 
      Process.send(pid, :msg)
   ]) do
   do_smt()

   assert_called MyModule.do_stm_else()
end

assert_receive(:msg)

Please note that I had no time to check this, you should spend a bit to investigate.