Fix connection-draining flakes in TestAllocatorAfterDeleteReplica / /TestGameServerCreationRightAfterDeletingOneExtensionsPod

[zmerlynn]

I've been fighting a 2-3% flake in these tests, both related to our servers not seemingly to terminate connections during graceful shutdown. Common errors include:

rpc error: code = Unavailable desc = error reading from server: EOF

and

rpc error: code = Unavailable desc = error reading from server: read tcp 192.168.25.113:36748->104.198.58.226:443: read: connection reset by peer

#3839 and #3843 were both towards fixing this but do not seem to have succeeded.