quic: fix flaky gso test

[danzh2010]

Commit Message: potential fix to flakiness caused by missing Read event when I/O event callback is triggered.

test/common/network/udp_listener_impl_test.cc:610: Failure
Actual function call count doesn't match EXPECT_CALL(listener_callbacks_, onReadReady())...
         Expected: to be called once
           Actual: never called - unsatisfied and active

This indicates Read event is not present when the test is unblocked. Maybe a Write only event is bubbled up at the beginning.

Risk Level: low, test only
Testing: existing tests pass
Docs Changes: N/A
Release Notes: N/A
Platform Specific Features: N/A
Fixes #29310

[danzh2010]

/retest

[danzh2010]

/retest