ppp-over-tls tests: try to keep CentOS 6 CI working, and improve flaky startup of...

ppp-over-tls tests: try to keep CentOS 6 CI working, and improve flaky startup of pppd

Even with EPEL, CentOS has an old version of socat which doesn't support the
'rawer' option, so let's use the older 'raw,echo=0' combination to keep it
limping along.

More carefully try to verify that socat and pppd start up and connect to each other:

- Wait for socat to create PTY in 1-second increments, and keep going until PTY
  actually exists (up to 15 seconds).
- Wait for ppp to connect to PTY in 1-second increments, and keep going until pppd
  creates a "UUCP-style lockfile" for the PTY.
- Log how long it takes for the above process to complete (socat and pppd combined
  startup) in the test output.

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