ccl/sqlproxyccl: TestProxyProtocol failed

ccl/sqlproxyccl.TestProxyProtocol [failed](https://teamcity.cockroachdb.com/buildConfiguration/Cockroach_Ci_TestsIbmCloudLinuxS390x_UnitTestsS390x/21187502?buildTab=log) with [artifacts](https://teamcity.cockroachdb.com/buildConfiguration/Cockroach_Ci_TestsIbmCloudLinuxS390x_UnitTestsS390x/21187502?buildTab=artifacts#/) on release-26.1.1-rc @ [5abbbd24fcc2c7873fb0f985ccea68b7abaa6afe](https://github.com/cockroachdb/cockroach/commits/5abbbd24fcc2c7873fb0f985ccea68b7abaa6afe):

```
=== RUN TestProxyProtocol
test_log_scope.go:171: test logs captured to: /artifacts/tmp/_tmp/be1ed5c5b909a51f68d3aca63bb5dc4d/logTestProxyProtocol2806701915
test_log_scope.go:82: use -show-logs to present logs inline
proxy_handler_test.go:2923:
Error Trace: pkg/ccl/sqlproxyccl/proxy_handler_test.go:2950
Error: Expected nil, but got: &sqlproxyccl.errWithCode{
code: 4,
cause: &withstack.withStack{
cause: &errutil.withPrefix{
cause: &errors.errorString{s:"invalid length of startup packet: 369295617"},
prefix: "while receiving startup message",
},
stack: &withstack.stack{0x85618d43, 0x8562865c, 0x8562fda8, 0x81aa6a2c, 0x800c3bb2},
},
}
Test: TestProxyProtocol
panic.go:615: -- test log scope end --
test logs left over in: /artifacts/tmp/_tmp/be1ed5c5b909a51f68d3aca63bb5dc4d/logTestProxyProtocol2806701915
--- FAIL: TestProxyProtocol (1.89s)
=== RUN TestProxyProtocol/server_doesn't_require_proxy_protocol
testing.go:1811: test executed panic(nil) or runtime.Goexit: subtest may have called FailNow on a parent test
--- FAIL: TestProxyProtocol/server_doesn't_require_proxy_protocol (1.09s)
```
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-61034