[vs test bug fix] Restore eth0 up in post-test clean up

[wendani]

As per #1030 (comment)

Signed-off-by: Wenda Ni wonda.ni@gmail.com

What I did

Why I did it
test_port.py test failure if follows test_portchannel.py using persistent vs container

$ sudo pytest -v --dvsname=vs -s test_portchannel.py 
================================================================ test session starts =================================================================
platform linux2 -- Python 2.7.13, pytest-4.6.9, py-1.8.1, pluggy-0.13.1 -- /usr/bin/python2
cachedir: .pytest_cache
rootdir: /home/wendani/sonic-buildimage-201911/src/sonic-swss/tests
plugins: flaky-3.7.0
collected 2 items                                                                                                                                    

test_portchannel.py::TestPortchannel::test_Portchannel remove extra link dummy
PASSED
test_portchannel.py::TestPortchannel::test_Portchannel_oper_down PASSED

$ sudo ip netns exec sw-srv0 ip -d link show eth0
1408: eth0@if1407: <BROADCAST,MULTICAST> mtu 1500 qdisc noqueue state DOWN mode DEFAULT group default qlen 1000
    link/ether e2:62:28:06:a9:84 brd ff:ff:ff:ff:ff:ff link-netnsid 1 promiscuity 0 
    veth addrgenmode eui64 numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535 

$ sudo pytest -v --dvsname=vs -s test_port.py
================================================================ test session starts =================================================================
platform linux2 -- Python 2.7.13, pytest-4.6.9, py-1.8.1, pluggy-0.13.1 -- /usr/bin/python2
cachedir: .pytest_cache
rootdir: /home/wendani/sonic-buildimage-201911/src/sonic-swss/tests
plugins: flaky-3.7.0
collected 6 items                                                                                                                                    

test_port.py::TestPort::test_PortMtu remove extra link dummy
PASSED
test_port.py::TestPort::test_PortNotification FAILED
test_port.py::TestPort::test_PortFec FAILED
test_port.py::TestPort::test_PortPreemp PASSED
test_port.py::TestPort::test_PortIdriver PASSED
test_port.py::TestPort::test_PortIpredriver PASSED

How I verified it
201911 branch

$ sudo pytest -v --dvsname=vs -s test_portchannel.py 
================================================================ test session starts =================================================================
platform linux2 -- Python 2.7.13, pytest-4.6.9, py-1.8.1, pluggy-0.13.1 -- /usr/bin/python2
cachedir: .pytest_cache
rootdir: /home/wendani/sonic-buildimage-201911/src/sonic-swss/tests
plugins: flaky-3.7.0
collected 2 items                                                                                                                                    

test_portchannel.py::TestPortchannel::test_Portchannel remove extra link dummy
PASSED
test_portchannel.py::TestPortchannel::test_Portchannel_oper_down PASSED

$ sudo ip netns exec sw-srv0 ip -d link show eth0
1408: eth0@if1407: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default qlen 1000
    link/ether e2:62:28:06:a9:84 brd ff:ff:ff:ff:ff:ff link-netnsid 1 promiscuity 0 
    veth addrgenmode eui64 numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535 

$ sudo pytest -v --dvsname=vs -s test_port.py 
================================================================ test session starts =================================================================
platform linux2 -- Python 2.7.13, pytest-4.6.9, py-1.8.1, pluggy-0.13.1 -- /usr/bin/python2
cachedir: .pytest_cache
rootdir: /home/wendani/sonic-buildimage-201911/src/sonic-swss/tests
plugins: flaky-3.7.0
collected 6 items                                                                                                                                    

test_port.py::TestPort::test_PortMtu remove extra link dummy
PASSED
test_port.py::TestPort::test_PortNotification PASSED
test_port.py::TestPort::test_PortFec PASSED
test_port.py::TestPort::test_PortPreemp PASSED
test_port.py::TestPort::test_PortIdriver PASSED
test_port.py::TestPort::test_PortIpredriver PASSED

============================================================= 6 passed in 55.92 seconds ==============================================================

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