teatest: fix race condition

[kalensk]

I am running into a race condition where the final model would still be nil rather than the expected model even after the program completed the the done signal sent. This seems to fix it.

Send the final model ‘before’ sending the program is done signal, such that FinalModel() can correctly receive the final model from modelCh rather than returning the default tm.model which is null.

func (tm *TestModel) FinalModel(tb testing.TB, opts ...FinalOpt) tea.Model {
	tm.waitDone(tb, opts)
	select {
	case m := <-tm.modelCh:
		tm.model = m
		return tm.model
	default:
		return tm.model
	}
}

Specifically, in the above code tm.waitDone(tb, opts) would wait for doneCh to receive true before selecting on the tm.modelCh which might not receive the final model since it is being sent 'after' the doneCh signal is sent. This fixes that ordering issue.

cc: @aymanbagabas

[kalensk]

@caarlos0 Let me know if this makes sense or any other questions. Happy to try and help.

[aymanbagabas]

Could you write a test case covering this one?

[kalensk]

Could you write a test case covering this one?

Unfortunately, I can't think of a test that would catch this without adding undo time to the tests or without other negative side effects. Can you?