networkd: print a meaningful error on failure

networkd: print a meaningful error on failure

test_keep_configuration_on_restart() works, but the error printed is
misleading because self.assertNotEmpty() doesn't exist.

Add a working assert statement so, when the unmanaged interface is
altered, the test fails with a meaningful error, like:

        ### ip monitor dev unmanaged0 BEGIN
        222:33::/64 proto kernel metric 256 pref medium
        FAIL
        [...]
        Traceback (most recent call last):
          File "/work/src/test/test-network/systemd-networkd-tests.py", line 5085, in test_keep_configuration_on_restart
            self.assertEqual(line, '')
        AssertionError: '222:33::/64 proto kernel metric 256 pref medium' != ''
        - 222:33::/64 proto kernel metric 256 pref medium

While at it, strip the trailing newline so we can print easily the
string (and in future build more a robust regexp which uses the $ token)