[32-74] FlowSplitWhenSpec (#6579)

* [32-74] `FlowSplitWhenSpec`

* Changes to `async` TestKit

* fix: `SplitWhen_must_work_in_the_happy_case`

* Revert "fix: `SplitWhen_must_work_in_the_happy_case`"

This reverts commit f4a29d3.

* Changes

* Fix

* fixed ` Func<Task<Source<int, NotUsed>>>> run`

* Changes

* changes it back into `WithSubstreamsSupport`

* Changes in `SplitWhen_must_work_in_the_happy_case`

* Changes to `WithSubstreamsSupportAsync`

* Revert "Changes to `WithSubstreamsSupportAsync`"

This reverts commit 5be01a9.

* Revert "Revert "Changes to `WithSubstreamsSupportAsync`""

This reverts commit c5bdb2a.

* Changes to `CreateManualSubscriberProbe<int>`

* Revert "Changes to `CreateManualSubscriberProbe<int>`"

This reverts commit 357f476.

* Revert "Revert "Revert "Changes to `WithSubstreamsSupportAsync`"""

This reverts commit 67df2f5.

* Change it back to `void`

* revert `https://github.com/akkadotnet/akka.net/pull/6579/commits/ca4b487fa61b50c9b8bb35239a72fd2aebd4bc62`

Author: eaba