Websocket sandbox test is flakey

[phlax]

description

The websocket test fail on some PRs for no immediately discernible reason

example failing logs (copied from slack)

2021-02-04T11:03:38.1149607Z > [tls] Interact with web socket ws -> ws
2021-02-04T11:03:38.3143654Z spawn docker run --rm -ti --network=host solsson/websocat ws://localhost:10000
2021-02-04T11:03:38.5956163Z websocat: WebSocketError: I/O failure
2021-02-04T11:03:38.5957714Z websocat: error running
2021-02-04T11:03:38.5967854Z HELO
2021-02-04T11:03:38.6510123Z send: spawn id exp3 not open
2021-02-04T11:03:38.6511546Z     while executing
2021-02-04T11:03:38.6512455Z "send \x03"
2021-02-04T11:03:38.6542212Z 
2021-02-04T11:03:38.6543324Z > [tls] Cleanup (.)

PRs known to have failing tests #14907 #14915

[phlax]

possible related vi/websocat#23 (comment)