Use WaitUntil in EndToEndTest.SimpleStopSendingTest to fix the flakiness.

PiperOrigin-RevId: 310198911
Change-Id: I67a0131c145897e2ddc8a28b8fda579804107d53

quic/core/http/end_to_end_test.cc

diff --git a/quic/core/http/end_to_end_test.cc b/quic/core/http/end_to_end_test.cc
index ccbe3af..7490e28 100644
--- a/quic/core/http/end_to_end_test.cc
+++ b/quic/core/http/end_to_end_test.cc
@@ -4016,7 +4016,8 @@
   // Ensure the stream has been write closed upon receiving STOP_SENDING.
   EXPECT_EQ(stream_id, client_stream->id());
   EXPECT_TRUE(client_stream->write_side_closed());
-  EXPECT_LT(0u, observer.num_stop_sending_frames());
+  client_->WaitUntil(
+      -1, [&observer]() { return observer.num_stop_sending_frames() > 0; });
   if (!observer.stream_write_side_closed_before_receiving_stop_sending()) {
     EXPECT_EQ(kStopSendingTestCode,
               static_cast<uint16_t>(client_stream->stream_error()));