failure in ud/test_ud_slow_timer.retransmit1/1 with valgrind

[yosefe]

http://hpc-master.lab.mtl.com:8080/job/hpc-ucx-pr/3818/label=hpc-test-node2,worker=3/console

01:11:16.830 [ RUN      ] ud/test_ud_slow_timer.retransmit1/1
01:11:17.776 /scrap/jenkins/scrap/workspace/hpc-ucx-pr/label/hpc-test-node2/worker/3/contrib/../test/gtest/uct/test_ud_slow_timer.cc:110: Failure
01:11:17.776 Value of: ucs_frag_list_sn(&ep(m_e2)->rx.ooo_pkts)
01:11:17.776   Actual: 0
01:11:17.776 Expected: 1
01:11:18.030 
[  FAILED  ] ud/test_ud_slow_timer.retransmit1/1, where GetParam() = ud/mlx5_0:1 (1195 ms)
...
01:18:37.363 [  PASSED  ] 1612 tests.
01:18:37.363 [  FAILED  ] 1 test, listed below:
01:18:37.363 [  FAILED  ] ud/test_ud_slow_timer.retransmit1/1, where GetParam() = ud/mlx5_0:1
01:18:37.363 
01:18:37.363  1 FAILED TEST
01:18:39.712 
make: *** [test_valgrind] Error 1

[yosefe]

fixed by #1635