selftests/net: Adding test cases of replacing routes and route advertisements.

Add tests of changing permanent routes to temporary routes and the reversed
case to make sure GC working correctly in these cases.  Add tests for the
temporary routes from RA.

The existing device will be deleted between tests to remove all routes
associated with it, so that the earlier tests don't mess up the later ones.

Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Tested-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>