UT in package net is always killed with -race

[djshow832]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Add -race to run go test in CI.

2. What did you expect to see? (Required)

No error

3. What did you see instead (Required)

The package net is always killed, even if I added a long timeout to go test.

e.g. https://do.pingcap.net/jenkins/blue/organizations/jenkins/pingcap%2Ftiproxy%2Fghpr_unit_test/detail/ghpr_unit_test/29/pipeline

It only runs for 36s, so it definitely not because of timeout (maybe because of OOM?):

signal: killed

FAIL	github.com/pingcap/tiproxy/pkg/proxy/net	36.001s

4. What is your version? (Required)

master