[3.4] Test failure for TestRoundRobinBalancedResolvableFailoverFromServerFail

[ahrtr]

{"level":"debug","msg":"picked","picker":"picker-roundrobin-balanced","address":"127.0.0.1:39987","subconn-index":0,"subconn-size":1}
{"level":"info","msg":"state changed","picker":"picker-roundrobin-balanced","balancer-id":"cld97ag39nv7","connected":false,"subconn":"0xc000316650","subconn-size":5,"address":"127.0.0.1:39987","old-state":"READY","new-state":"CONNECTING"}
{"level":"info","msg":"updated picker","picker":"picker-roundrobin-balanced","balancer-id":"cld97ag39nv7","policy":"picker-roundrobin-balanced","subconn-ready":[],"subconn-size":0}
{"level":"warn","msg":"balancer failed","error":"rpc error: code = Unavailable desc = transport is closing","picker":"picker-roundrobin-balanced","address":"127.0.0.1:39987","success":false,"bytes-sent":true,"bytes-received":false}
--- FAIL: TestRoundRobinBalancedResolvableFailoverFromServerFail (0.01s)
    balancer_test.go:167: Unexpected failure rpc error: code = Unavailable desc = transport is closing

Also refer to https://github.com/etcd-io/etcd/runs/7293195456?check_suite_focus=true

[ahrtr]

cc @lavacat

[lavacat]

/assign

[lavacat]

PR #14274

[ahrtr]

Fixed by #14274