stream_op cleanup: test fixes and updates

test/core/channel/channel_stack_test.c

test/core/client_config/lb_policies_test.c

test/core/end2end/fixtures/proxy.c

test/core/end2end/tests/max_message_length.c

test/core/end2end/tests/request_with_flags.c

test/core/network_benchmarks/low_level_ping_pong.c

test/core/transport/chttp2/hpack_encoder_test.c

test/core/transport/stream_op_test.c

test/cpp/end2end/end2end_test.cc

test/cpp/interop/interop_client.cc