test(server): test server keep alive by counting number of pings
test(server): test server keep alive by counting number of pings

Add `http2_keep_alive_count_server_pings` which tests if 3 pings are
received under 5 seconds with keep alive interval set to 1 second.

hyperium/hyper,tshepang/hyper,tshepang/hyper,hyperium/hyper,tshepang/hyper,hyperium/hyper