Race condition in "query" tests

[wingo]

For horrible reasons, Snabb counters have two lives: an existence that's local to the process, on the Lua heap, and an external copy that's in shared memory. Snabb will periodically "commit" the private counter values to the public locations. This means there can be a delay between a counter being updated privately and the new value being publically visible. This is a race condition that can cause tests to fail, if they assume that they can perform some sequence of actions and then use snabb lwaftr query or snabb config get-state to read counter values. We should fix this somehow!

[dpino]

Perhaps unrelated but worth mentioning there was a race condition, due to missing a signal when parent process died, that affected query and other subcommands. Fixed in #1002