Use NiceMock for MockRandom to fix test failures in Envoy.

I encountered this issue when merging cl/473306804 into Envoy.
It seems that this change somehow caused the mocked object to be strict. And the lack of EXPECT_CALL() fails many tests.

I don't know why this is not an issue in Google3 though.

PiperOrigin-RevId: 474050286

quiche/quic/test_tools/quic_test_utils.h

diff --git a/quiche/quic/test_tools/quic_test_utils.h b/quiche/quic/test_tools/quic_test_utils.h
index 57eb068..7981335 100644
--- a/quiche/quic/test_tools/quic_test_utils.h
+++ b/quiche/quic/test_tools/quic_test_utils.h
@@ -527,7 +527,7 @@
 
  private:
   MockClock clock_;
-  MockRandom random_generator_;
+  testing::NiceMock<MockRandom> random_generator_;
   quiche::SimpleBufferAllocator buffer_allocator_;
 };