test/system: Add tests for pasta(1) connectivity

These tests should cover all the basic networking functionality with
pasta(1). Namely, they check:

- IPv4 and IPv6 addressing and routing settings

- TCP and UDP port forwarding over IPv4 and IPv6

- data transfers and ICMP/ICMPv6 echo requests

- the (exceedingly simple) lifecycle handling

These tests need some new helpers, to obtain IPv4 and IPv4 addresses
and routes, as well as MTU and interface names. Those use jq(1) for
parsing.

Some availability checks are implemented as well, to skip tests if
pasta(1) is not available, or if IPv4 and IPv6 are not usable.

To get consistent outcomes across distributions, and to enable
uncomplicated termination for UDP tests based on zero-sized packets,
use socat(1), which, unlike netcat, doesn't suffer from option
inconsistencies depending on flavours (traditional, BSD, NMAP) and
versions.

Signed-off-by: Stefano Brivio <[email protected]>