Integration tests should report warnings/errors in logs

[mum4k]

This would have given us an earlier warning about the Envoy v2 API deprecation. An example of an error message that is logged by the nighthawk test server:

[2020-12-01 01:39:50.785][114][warning][misc] [external/envoy/source/common/protobuf/utility.cc:294] Configuration does not parse cleanly as v3. v2 configuration is deprecated and will be removed from Envoy at the start of Q1 2021: Unknown field in: {"admin":{"profile_path":"/tmp/2020-12-01-01-39-49/http_h1_connection_management_single_request_per_conn_1_IpVersion.IPV6//nighthawk-test-server.prof","access_log_path":"/tmp/2020-12-01-01-39-49/http_h1_connection_management_single_request_per_conn_1_IpVersion.IPV6//nighthawk-test-server-admin-access.log","address":{"socket_address":{"address":"::/0","port_value":0}}},"static_resources":{"listeners":[{"filter_chains":[{"filters":[{"config":{"generate_request_id":false,"codec_type":"auto","route_config":{"virtual_hosts":[{"name":"service","domains":["*"]}],"name":"local_route"},"http_filters":[{"name":"time-tracking"},{"name":"dynamic-delay"},{"name":"test-server","config":{"response_body_size":10,"response_headers":[{"header":{"key":"x-nh","value":"1"}}]}},{"name":"envoy.router","config":{"dynamic_stats":false}}],"stat_prefix":"ingress_http"},"name":"envoy.http_connection_manager"}]}],"address":{"socket_address":{"port_value":0,"address":"::/0"}}}]}}
[2020-12-01 01:39:50.787][114][warning][misc] [external/envoy/source/common/protobuf/message_validator_impl.cc:21] Deprecated field: type envoy.api.v2.listener.Filter Using deprecated option 'envoy.api.v2.listener.Filter.config' from file listener_components.proto. This configuration will be removed from Envoy soon. Please see https://www.envoyproxy.io/docs/envoy/latest/version_history/version_history for details. If continued use of this field is absolutely necessary, see https://www.envoyproxy.io/docs/envoy/latest/configuration/operations/runtime#using-runtime-overrides-for-deprecated-features for how to apply a temporary and highly discouraged override.

Ideally the implementation would support some kind of list to allow us to "acknowledge" or "filter" known warnings if any.