=con #15613 Rename one of the DistributedPubSubMediatorSpec classes

(cherry picked from commit 4c4b1a1d2732eb01c0b519cf55ef877a76d3035a)
Merge pull request #15636 from akka/wip-15613-rename-DistributedPubSubMediatorSpec-2.3-patriknw

=con #15613 Rename one of the DistributedPubSubMediatorSpec classes (for verification)