add ppp-over-tls tests (with pppd as the reference peer implementation)

add ppp-over-tls tests (with pppd as the reference peer implementation)

These test OpenConnect's ability to communicate with the standard pppd using
PPP-over-TLS, with a variety of PPP negotiation options:

  ± IPv4
  ± IPv6
  ± DNS and NBNS server negotiation
  ± PPP header protocol/address field compression
  ± Van Jacobson header compression (always rejected by OpenConnect)

These tests use socat to create TLS socket pairs, connecting `openconnect
--protocol=nullppp` to one end and `pppd` to the other.

I tried and failed to combine socat and pppd invocations, but pppd seemingly
cannot handle being wrapped by libsocket_wrapper.so (nor libuid_wrapper.so;
it must run as root).

`pppd sync` (non-HDLC framing) appears to have trouble reacting to incoming
packets in this configuration, so OpenConnect has to invoke retry timers during
negotiation. This seems to be a bug in pppd, rather than a flaw in OpenConnect's
implementation of PPP. Added logging of the time that each run takes.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>