Add missing specs for Host

The following methods are missing specs:
- `#mac` (requires `//host/address[@addrtype='mac']`)
- `#ipv6` (requires `//host/address[@addrtype='ipv6']`)
- `#each` (just calls `each_open_port`)
- `#scripts` (requires //host/hostscript/script`)
- `#traceroute` (requires `//host/trace`)
