How to write a simple unit test for GraphQL Subscription over websocket?

My backend is a webflux server using the graphql-java-kickstart (v11.0.0) library and I have a bunch of graphql http requests right now.

I'm using Spring's WebTestClient to test my graphql Queries and Mutations. Basically, I'll just call it and compare the response like so

webTestClient
  .post()
  .uri("/graphql")
  .header(...)
  .body(Mono.just(request), JsonNode.class)
  .exchange()
  .expectStatus()
  .isOk()
  .expectBody(JsonNode.class)
  .returnResult()
  .getResponseBody()

But this is over a normal HTTP and simple REST protocol.

With GraphQL Subscriptions, I need to set up a websocket and do the Apollo (subscription-transport.ws) handshake protocol before sending the Subscription request in, and I have to wait for a response async.

I have manually verified the subscription works with the WebSocket Test Client chrome extension

What's the correct way to write a unit test for this?

Suppose I have a simple Subscription that just returns the timestamp back every second

type Subscription {
  clock: String!
}

How do I write a unit test for this? What library or client should I use? A link or example would be awesome!