* op-node: p2p pinging background service

* op-node: remove p2p ping flag whitespace
Co-authored-by: Joshua Gutow <jgutow@oplabs.co>

* op-node: implement ping-service review suggestions

* op-node: test case numbers in the require msg-args, not the log filter itself

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Co-authored-by: Joshua Gutow <jgutow@oplabs.co>