This commit adds support for IPv6 loopback address (::1) in the
simulation platform. When the local interface is set to the IPv6
loopback address, it uses the interface-local multicast group
(ff01::116) instead of the link-local group (ff02::116) for
node-to-node communication.
It also ensures that the `sin6_scope_id` is correctly set for the
loopback address in the transmission socket.
This commit adds default paths for expect scripts to make it easier for
running expect scripts. With this change, we can run expect tests as
follows:
```bash
./script/cmake-build simulation
./script/cmake-build posix
./tests/scripts/expect/cli-ping.exp
OT_NODE_TYPE=rcp ./tests/scripts/expect/cli-ping.exp
```
This commit also updates an existing test to cover the change.
This commit adds support to simulate Thread radio over IPv6.
With this commit, a simulation will be simulated over either IPv6 or IPv4.
If it's simulated on IPv6, it communicates with other simulation nodes
in IPv6 group `ff02::116`. And if it's simulated on IPv4, it communicates
with other simulation nodes in IPv4 group `224.0.0.116`.
Note that simulating virtual time is not included in this commit.