### Add ability to specify multi-vnet topologies for the testing framework.
Notable amount of tests related to the packet IO require two VNET jails for proper testing and avoiding side effects for the host system. Additionally, it is often required to run actions in the jails some-sequentially - waiting for the listener initialisation can be an example.
This change extends pytest vnet framework to allow defining multi-vnet multi-epair topologies in declarative style, without any need to bother about jail or repair names. All jail creation/teardown, interface creation/teardown and address assignments are handled automatically.
Example:
```
TOPOLOGY = {
"vnet1": {"ifaces": ["if1", "if2", "if3"]},
"vnet2": {"ifaces": ["if1", "if2", "if3"]},
"if1": {"prefixes6": [("2001:db8:a::1/64", "2001:db8:a::2/64")]},
"if2": {"prefixes6": [("2001:db8:b::1/64", "2001:db8:b::2/64")]},
"if3": {"prefixes6": [("2001:db8:c::1/64", "2001:db8:c::2/64")]},
}
def vnet2_handler(self, vnet, obj_map, pipe):
ss = VerboseSocketServer("::", self.DEFAULT_PORT)
pipe.send("READY")
def test_output6_base(self):
self.wait_object(second_vnet.pipe)
```
The definitions above will create 2 vnets ("jail_test_output6_base", "jail_test_output6_base_2"), 3 epairs, attached to both first and second jails, set up the IP addresses for each repair, spawn another process for `vnet2_handler` and pass control to `vnet2_handler` and `test_output6_base`. Both processes can pass objects between each other using pre-created pipes.
### Add ip6_output() tests
Using the framework above, add tests for ip6_output() combined with the unicast/multicast options, altering packet behaviour.