`http2_general.test_h2_frame.TestPostponedFrames` don't work on CI

[RomanBelozerov]

These tests fail on CI in most cases. (for master and release-0.8)

master:
Tempesta - 5a4f1c7ae61c00490c14e0b89738e0b96d3b415d
kernel-version - 6.12.12.tfw-1493f1e
tempesta-test - 36b6532

release-0.8:
Tempesta - ed730df00baf2b10046bea6b5e36cc435b7a099d
kernel version - 5.10.35.tfw-562f4af
tempesta-test - 1c059bf

Traceback

======================================================================
FAIL: test_headers (http2_general.test_h2_frame.TestPostponedFrames)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/jenkins/workspace/Daily_tests_release/tempesta-test/venv/lib/python3.10/site-packages/parameterized/parameterized.py", line 620, in standalone_func
    return func(*(a + p.args), **p.kwargs, **kw)
  File "/home/jenkins/workspace/Daily_tests_release/tempesta-test/helpers/networker.py", line 65, in func_wrapper
    return NetWorker().__protect_ipv6_addr_on_dev(func, *args, **kwargs)
  File "/home/jenkins/workspace/Daily_tests_release/tempesta-test/helpers/networker.py", line 55, in __protect_ipv6_addr_on_dev
    return func(*args, **kwargs)
  File "/home/jenkins/workspace/Daily_tests_release/tempesta-test/http2_general/test_h2_frame.py", line 944, in test
    self.run_test_tso_gro_gso_disabled(client, server, self._test, 100)
  File "/home/jenkins/workspace/Daily_tests_release/tempesta-test/helpers/networker.py", line 151, in run_test_tso_gro_gso_disabled
    self.run_test_tso_gro_gso(client, server, test, mtu, False, False, False)
  File "/home/jenkins/workspace/Daily_tests_release/tempesta-test/helpers/networker.py", line 140, in run_test_tso_gro_gso
    test(client, server)
  File "/home/jenkins/workspace/Daily_tests_release/tempesta-test/http2_general/test_h2_frame.py", line 905, in _test
    self.assertTrue(client.wait_for_ping_frames(ping_count))
AssertionError: False is not true