Fix flaky //third_party/quic/core/batch_writer:quic_batch_writer_test

```
blaze test //third_party/quic/core/batch_writer:quic_batch_writer_test --runs_per_test=10000

//third_party/quic/core/batch_writer:quic_batch_writer_test              PASSED in 63.2s
  Stats over 10000 runs: max = 63.2s, min = 1.1s, avg = 2.8s, dev = 1.3s

Executed 1 out of 1 test: 1 test passes.
INFO: Streaming build results to: http://sponge2/630c0dde-180e-49c0-ae94-1ec704e5154c
```
PiperOrigin-RevId: 605628040

quiche/quic/core/batch_writer/quic_batch_writer_test.h

diff --git a/quiche/quic/core/batch_writer/quic_batch_writer_test.h b/quiche/quic/core/batch_writer/quic_batch_writer_test.h
index 7fa200d..71c59b3 100644
--- a/quiche/quic/core/batch_writer/quic_batch_writer_test.h
+++ b/quiche/quic/core/batch_writer/quic_batch_writer_test.h
@@ -231,6 +231,8 @@
       QuicUdpSocketApi::ReadPacketResult result;
       result.packet_buffer = {&packet_buffer_[0], sizeof(packet_buffer_)};
       result.control_buffer = {&control_buffer_[0], sizeof(control_buffer_)};
+      ASSERT_TRUE(QuicUdpSocketApi().WaitUntilReadable(
+          peer_socket_, QuicTime::Delta::FromSeconds(1)));
       QuicUdpSocketApi().ReadPacket(
           peer_socket_,
           quic::QuicUdpPacketInfoBitMask({QuicUdpPacketInfoBit::V4_SELF_IP,