sqlproxy: TestProxyProtocol failed

sqlproxy.TestProxyProtocol [failed](https://teamcity.cockroachdb.com/buildConfiguration/Cockroach_Ci_TestsIbmCloudLinuxS390x_UnitTestsS390x/21366819?buildTab=log) with [artifacts](https://teamcity.cockroachdb.com/buildConfiguration/Cockroach_Ci_TestsIbmCloudLinuxS390x_UnitTestsS390x/21366819?buildTab=artifacts#/) on master @ [84af112a1d114f3ac3a163101266b663c7149263](https://github.com/cockroachdb/cockroach/commits/84af112a1d114f3ac3a163101266b663c7149263):

```
=== RUN TestProxyProtocol
test_log_scope.go:172: test logs captured to: /artifacts/tmp/_tmp/5da4eb2a0833eab61100a8e075aa4b75/logTestProxyProtocol673344868
test_log_scope.go:82: use -show-logs to present logs inline
test_server_shim.go:637: DRPC is enabled (via testserver factory defaults)
proxy_handler_test.go:2922:
Error Trace: pkg/sqlproxy/proxy_handler_test.go:2949
Error: Expected nil, but got: &sqlproxy.errWithCode{
code: 4,
cause: &withstack.withStack{
cause: &errutil.withPrefix{
cause: &errors.errorString{s:"invalid length of startup packet: 1146245183"},
prefix: "while receiving startup message",
},
stack: &withstack.stack{0x84828e97, 0x84836e2a, 0x8483d8aa, 0x818d3710, 0x800c4ef2},
},
}
Test: TestProxyProtocol
panic.go:694: -- test log scope end --
test logs left over in: /artifacts/tmp/_tmp/5da4eb2a0833eab61100a8e075aa4b75/logTestProxyProtocol673344868
--- FAIL: TestProxyProtocol (6.83s)
=== RUN TestProxyProtocol/server_doesn't_require_proxy_protocol
testing.go:1913: test executed panic(nil) or runtime.Goexit: subtest may have called FailNow on a parent test
--- FAIL: TestProxyProtocol/server_doesn't_require_proxy_protocol (1.10s)
```
Help

See also: [How To Investigate a Go Test Failure \(internal\)](https://cockroachlabs.atlassian.net/l/c/HgfXfJgM)

/cc @cockroachdb/sqlproxy-prs

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Jira issue: CRDB-64427