VPP-1486: Unittest for stat segment file descriptor leak.

VPP-1486: Unittest for stat segment file descriptor leak.

Verifies: https://gerrit.fd.io/r/#/c/18167/

Before patch:

    ==============================================================================
    Test Stats Client
    ==============================================================================
    Test file descriptor count - VPP-1486                                    FAIL [ temp dir used by test case: /tmp/vpp-unittest-StatsClientTestCase-EAp0e7 ]

    ==============================================================================
    FAIL: Test file descriptor count - VPP-1486
    ------------------------------------------------------------------------------
    Traceback (most recent call last):
      File "/vpp/test/test_stats_client.py", line 39, in test_client_fd_leak
        initial_fds, ending_fds))
    AssertionError: initial client side file descriptor count: 20 is not equal to ending client side file descriptor count: 120

    04:55:38,038 Symlink to failed testcase directory: /tmp/vpp-failed-unittests/vpp-unittest-StatsClientTestCase-EAp0e7-FAILED -> vpp-unittest-StatsClientTestCase-EAp0e7

    ==============================================================================
    TEST RESULTS:
         Scheduled tests: 1
          Executed tests: 1
            Passed tests: 0
                Failures: 1
    FAILURES AND ERRORS IN TESTS:
      Testcase name: Test Stats Client
        FAILURE: Test file descriptor count - VPP-1486 [test_stats_client.StatsClientTestCase.test_client_fd_leak]
    =============================================================================

After patch:

    ==============================================================================
    Test Stats Client
    ==============================================================================
    Test file descriptor count - VPP-1486                                    OK

    ==============================================================================
    TEST RESULTS:
         Scheduled tests: 1
          Executed tests: 1
            Passed tests: 1
    ==============================================================================

    Test run was successful

Change-Id: I055e473ecf0566ebfbfbadd58ec6eaf11fc77d68
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>