[BGP] Add clear arp to fix ping error in test_bgp_speaker

[bingwang-ms]

Description of PR

Summary:
This commit is to fix the exception when pinging from DUT in test_bgp_speaker.

E               "stdout": "PING 192.168.0.3 (192.168.0.3) 56(84) bytes of data.\n\n--- 192.168.0.3 ping statistics ---\n3 packets transmitted, 0 received, 100% packet loss, time 2034ms", 
E               "stdout_lines": [
E                   "PING 192.168.0.3 (192.168.0.3) 56(84) bytes of data.", 
E                   "", 
E                   "--- 192.168.0.3 ping statistics ---", 
E                   "3 packets transmitted, 0 received, 100% packet loss, time 2034ms"
E               ]

The ping command in test_bgp_speaker return error code when running on Jenkins. This is because the test_wr_arp leave ARP table on DUT, which causes some ip address unreachable. This commit add clear after test_wr_arp and before test_bgp_speaker

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• Test case(new/improvement)

Approach

What is the motivation for this PR?

This PR is to fix ping failure in test_bgp_speaker when running on Jenkins.

How did you do it?

1. Add sonic-clear arp after test_wr_arp to clear ARP cache, in case that the ARP table has lasting effect on other cases;
2. Add sonic-clear arp before test_bgp_speaker to make itself more robust.

How did you verify/test it?

1.Run test_wr_arp and test_bgp_speaker_bgp_sessions, and both cases passed.
2.Run show arp on DUT to confirm ARP table is cleared.

========================================================================================= test session starts =========================================================================================
platform linux2 -- Python 2.7.12, pytest-4.6.5, py-1.8.1, pluggy-0.13.1 -- /usr/bin/python
cachedir: .pytest_cache
ansible: 2.8.7
rootdir: /data/Networking-acs-sonic-mgmt/tests, inifile: pytest.ini
plugins: forked-1.1.3, xdist-1.28.0, ansible-2.2.2, repeat-0.8.0
collected 1 item                                                                                                                                                                                      

arp/test_wr_arp.py::TestWrArp::testWrArp ^@PASSED                                                                                                                                                 [100%]
platform linux2 -- Python 2.7.12, pytest-4.6.5, py-1.8.1, pluggy-0.13.1 -- /usr/bin/python
cachedir: .pytest_cache
ansible: 2.8.7
rootdir: /data/Networking-acs-sonic-mgmt/tests, inifile: pytest.ini
plugins: forked-1.1.3, xdist-1.28.0, ansible-2.2.2, repeat-0.8.0
collected 1 item                                                                                                                                                                                      

bgp/test_bgp_speaker.py::test_bgp_speaker_bgp_sessions PASSED                                                                                                                                   [100%]

Any platform specific information?

No.

Supported testbed topology if it's a new test case?

N/A

Documentation

N/A

[lolyu]

LGTM