Nightly: Add listening check on TCP KeepAlive
Nightly: Add listening check on TCP KeepAlive

The Nightly build [0] failed because a race conditicion, nc was not
listening yet and the test fail. With this change client execution waits
until NC is listening correctly.

Fix #3942

[0] https://jenkins.cilium.io/job/Cilium-Master-Nightly-Tests-All/165/

Signed-off-by: Eloy Coto <2a8acc28452926ceac4d9db555411743254cf5f9@gmail.com>

scanf/cilium,eloycoto/cilium,eloycoto/cilium,michi-covalent/cilium,eloycoto/cilium,cilium-team/cilium,scanf/cilium,eloycoto/cilium,michi-covalent/cilium,cilium/cilium,scanf/cilium,michi-covalent/cilium,tklauser/cilium,tgraf/cilium,tklauser/cilium,eloycoto/cilium,cilium-team/cilium,tklauser/cilium,scanf/cilium,scanf/cilium,tklauser/cilium,tgraf/cilium,michi-covalent/cilium,scanf/cilium,eloycoto/cilium,tgraf/cilium,scanf/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,cilium/cilium,eloycoto/cilium,tklauser/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium,michi-covalent/cilium