system-traffic: Wait for IPv6 connectivity.

system-traffic: Wait for IPv6 connectivity.

Several of the tests have race conditions where the next step in the
test may run before the kernel actually provides IPv6 connectivity.
This causes intermittent testsuite failures. Some existing tests
would even sleep in an attempt to mitigate this issue.

Improve the resilience of these tests by waiting until IPv6 or FTP
connectivity are ready. This speeds the testsuite up by a couple of
percent.

Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Jarno Rajahalme <jarno@ovn.org>