travis unittests: add more test cases

[joergsteffens]

Add more test cases and makes it possible to separately test client and server parts (by connecting against the openssl binary).

[joergsteffens]

My main reason for adding theses tests is that I run into problems on other Linux platforms than Ubuntu 16.04 used by Travis here.

In https://github.com/joergsteffens/sslpsk/tree/unittest-distris I added Ubuntu 18.04 and Ubuntu 20.04 to the platforms tested by Travis. Result: https://travis-ci.com/github/joergsteffens/sslpsk/builds/196602525 Summary:

• Ubunutu 18.04 (bionic)
  • client: ssl.PROTOCOL_TLS does not work.
  • ssl.SSLError: [SSL: ATTEMPT_TO_REUSE_SESSION_IN_DIFFERENT_CONTEXT] attempt to reuse session in different context (_ssl.c:1108)
  • server: all protocol setting do work.
• Ubuntu 20.04 (focal)
  • client: only ssl.PROTOCOL_TLSv1_2 does work.
  • ssl.PROTOCOL_TLS error:
    • ssl.SSLError: [SSL: ATTEMPT_TO_REUSE_SESSION_IN_DIFFERENT_CONTEXT] attempt to reuse session in different context (_ssl.c:1108)
  • else:
    • ssl.SSLError: [SSL] internal error (_ssl.c:1108)
  • server: only ssl.PROTOCOL_TLSv1_2 and ssl.PROTOCOL_TLS do work.
  • error:
    • [SSL] internal error (_ssl.c:1108)

Mac osx seams to have similar problems than Ubunutu 18.04.