Make lib/web/TestWesocketPingLoop more robust
Make lib/web/TestWesocketPingLoop more robust

Client can receive multiple raw output frames, if the shell prompt is
long. Instead of guessing the size of raw frames before pings start,
read all frames and count only the pings.

gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport