TestBackendTimeoutWithSlowBodyWriterShadow flakytest

[szuecs]

https://github.com/zalando/skipper/actions/runs/20748684995/job/59572170101

time="2026-01-06T12:56:17Z" level=error msg="error while proxying after 227.133µs, route groupA with backend lb http://127.0.0.1:40839, status code 502: dialing failed true: failed to do backend roundtrip to 127.0.0.1:40839: dial tcp 127.0.0.1:40839: connect: connection refused, remote host: 127.0.0.1, request: \"GET /a HTTP/1.1\", host: 127.0.0.1:41699, user agent: \"Go-http-client/1.1\""
--- FAIL: TestBackendTimeoutWithSlowBodyWriterShadow (1.22s)
    fifo_backendtimeout_with_teeloopback_test.go:94: metric "fifo.main.active": 1
    fifo_backendtimeout_with_teeloopback_test.go:94: metric "fifo.main.queued": 0
    fifo_backendtimeout_with_teeloopback_test.go:94: metric not found: "fifo.main.error.full"
    fifo_backendtimeout_with_teeloopback_test.go:94: metric not found: "fifo.main.error.other"
    fifo_backendtimeout_with_teeloopback_test.go:94: metric not found: "fifo.main.error.timeout"
    fifo_backendtimeout_with_teeloopback_test.go:94: metric "fifo.shadow.active": 1
    fifo_backendtimeout_with_teeloopback_test.go:94: metric "fifo.shadow.queued": 0
    fifo_backendtimeout_with_teeloopback_test.go:94: metric not found: "fifo.shadow.error.full"
    fifo_backendtimeout_with_teeloopback_test.go:94: metric not found: "fifo.shadow.error.other"
    fifo_backendtimeout_with_teeloopback_test.go:94: metric not found: "fifo.shadow.error.timeout"
    fifo_backendtimeout_with_teeloopback_test.go:99: response code: 200
    fifo_backendtimeout_with_teeloopback_test.go:105: Failed to get expected error: timeout waiting for log entry: failed to execute loopback request: dialing failed false: context deadline exceeded
time="2026-01-06T12:56:25Z" level=error msg="Failed to retry backend request: dialing failed true: failed to do backend roundtrip to [::]:40731: dial tcp [::]:40731: connect: connection refused"