+stk add TestSubscriber/Publisher.Probe.ensureSubscription to allow manual initialization

Otherwise, it is not possible to use `expectNoMsg()` as the first operation.

akka-stream-testkit/src/main/scala/akka/stream/testkit/StreamTestKit.scala

@@ -137,6 +137,9 @@ object TestPublisher {
     private var pendingRequests = initialPendingRequests
     private lazy val subscription = expectSubscription()
 
+    /** Asserts that a subscription has been received or will be received */
+    def ensureSubscription(): Unit = subscription // initializes lazy val
+
     /**
      * Current pending requests.
      */
@@ -558,6 +561,9 @@ object TestSubscriber {
 
     private lazy val subscription = expectSubscription()
 
+    /** Asserts that a subscription has been received or will be received */
+    def ensureSubscription(): Unit = subscription // initializes lazy val
+
     def request(n: Long): Self = {
       subscription.request(n)
       this