test-network: call networkctl only when specified interface exists

test-network: call networkctl only when specified interface exists

Otherwise, this easily trigger another exception:
```
======================================================================
ERROR: test_erspan_tunnel_v0 (__main__.NetworkdNetDevTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "./test/test-network/systemd-networkd-tests.py", line 686, in wait_online
    check_output(*args, env=env)
  File "./test/test-network/systemd-networkd-tests.py", line 65, in check_output
    return subprocess.check_output(command, universal_newlines=True, **kwargs).rstrip()
  File "/usr/lib64/python3.6/subprocess.py", line 356, in check_output
    **kwargs).stdout
  File "/usr/lib64/python3.6/subprocess.py", line 438, in run
    output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command '['/usr/lib/systemd/systemd-networkd-wait-online', '--timeout=20s', '--interface=erspan99:routable', '--interface=erspan98:routable', '--interface=dummy98:degraded']' returned non-zero exit status 1.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "./test/test-network/systemd-networkd-tests.py", line 1808, in test_erspan_tunnel_v0
    self.wait_online(['erspan99:routable', 'erspan98:routable', 'dummy98:degraded'])
  File "./test/test-network/systemd-networkd-tests.py", line 689, in wait_online
    output = check_output(*networkctl_cmd, '-n', '0', 'status', link.split(':')[0], env=env)
  File "./test/test-network/systemd-networkd-tests.py", line 65, in check_output
    return subprocess.check_output(command, universal_newlines=True, **kwargs).rstrip()
  File "/usr/lib64/python3.6/subprocess.py", line 356, in check_output
    **kwargs).stdout
  File "/usr/lib64/python3.6/subprocess.py", line 438, in run
    output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command '['/usr/bin/networkctl', '-n', '0', 'status', 'erspan99']' returned non-zero exit status 1.
```