Simulate Multiple Threads in Route Test

Not sure if this is the right question, but in my akka http-based system, when processing a request, we have calls that behave differently in RouteTests vs real systems. Given the following code:

  def fn(thing: Thing): EitherT[Future, Error, Thing] = {
    if (someCondition) {
    } else {

  def rollback(): EitherT[...] = {
    // general cleanup

  List(things).map(fn).sequence.leftMap {
    case err =>

In tests, the .sequence completion only ever happens after all the fn() executions complete, even if there is a failure.

In real life, a failure from one execution of fn() will cause .sequence to execute while other calls to fn() are still executing, which is undesired, but, afaik, expected. I want to test a workaround, but I am unable to simulate the real-world conditions inside a RouteTest, and I think this is because everything is running on one thread.

Is there a way to make the RouteTest simulate a multi-threaded scenario?