Add s2n Client Integration Test with Top N Domains

[alexw91]

Problem:
Right now we only test s2n's client implementation against s2n's server, OpenSSL server, and a few external domains here.

We'd like our client side integration tests to be more robust at catching some of the "long tail" of possible TLS compatibility bugs with more uncommon TLS libraries.

Proposed Solution:
Update s2n_client_endpoint_handshake_test.py to read domains to handshake with from a file (Eg top_10k_domains.txt) instead of the list of 8 domains currently used. We should be able to create a script to generate our own shortened list from free Top 1 million lists such as Alexa Top Sites or Cisco Top Million Domains.

By adding so many more domains to this test, we'll probably also need to implement additional checks to catch and ignore false positives. One simple check we could do would be to perform two TLS handshakes, one with OpenSSL client and one with s2n client, and only fail the integration test if OpenSSL is successful, but s2n fails or vice/versa.

[colmmacc]

I love this item. We also have an Amazon Web Service for this ... https://aws.amazon.com/alexa-top-sites/

[jmayclin]

We are cleaning up stale feature requests. Please reopen this issue if you believe it should be reprioritized.