test-network: wait a while for addresses to be dropped

test-network: wait a while for addresses to be dropped

Hopefully fixes the following failure:
```
======================================================================
FAIL: test_ipv6_token_prefixstable (__main__.NetworkdRATests.test_ipv6_token_prefixstable)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/systemd/tests/testdata/test-network/systemd-networkd-tests.py", line 5705, in test_ipv6_token_prefixstable
    self.assertNotIn('2002:da8:1:0:b47e:7975:fc7a:7d6e/64', output) # the 1st prefixstable
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: '2002:da8:1:0:b47e:7975:fc7a:7d6e/64' unexpectedly found in
  '19: veth99@veth-peer: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000\n
       inet6 2002:da8:1:0:b47e:7975:fc7a:7d6e/64 scope global tentative dynamic mngtmpaddr noprefixroute \n
          valid_lft 2100sec preferred_lft 1000sec\n
       inet6 2002:da8:1:0:da5d:e50a:43fd:5d0f/64 scope global dynamic mngtmpaddr noprefixroute \n
          valid_lft 2100sec preferred_lft 1000sec\n
       inet6 fe80::1034:56ff:fe78:9abc/64 scope link proto kernel_ll \n
          valid_lft forever preferred_lft forever'
----------------------------------------------------------------------
```