Fix flaky test
https://gitlab.com/gitlab-org/cluster-integration/gitlab-agent/-/jobs/1648998544#L1982
=== RUN TestRouter_StreamErrorAfterHeader
logger.go:130: 2021-10-05T04:00:49.699Z DEBUG Trying tunnel {"agent_id": 123, "connection_id": 1312312313, "kas_url": "grpc://pipe"}
router_test.go:197:
Error Trace: router_test.go:197
router_test.go:403
router_test.go:190
Error: Received unexpected error:
EOF
Test: TestRouter_StreamErrorAfterHeader
--- FAIL: TestRouter_StreamErrorAfterHeader (0.01s)