=con #15577 Harden ReliableProxyDocSpec

* I couldn't find anything wrong
* Increasing the test timeout, it takes 1.5 s for the reconnects,
  so the previous total of 3 s might not have been enough
  (for that run)
Merge pull request #15774 from akka/wip-15577-ReliableProxyDocSpec-patriknw

=con #15577 Harden ReliableProxyDocSpec