Avoid a race with test peer advert and DUT send

Fix a race where the DUT could send out test data before it received the
peer window advertisement. Such a race results in the DUT taking longer
time to retransmit zero window probe, thus causing the test to fail
receiving the last expected probe.

To ensure this ordering, piggyback a non-zero payload with the zero
window advertisement and let the DUT receive that, before continuing
with the test.

PiperOrigin-RevId: 361640241

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