Fix Flaky s2n_session_ticket_test

[alexw91]

Problem:

This test failed on this Travis Job: https://travis-ci.org/awslabs/s2n/jobs/652641395
But passed when the build was retried.

Running s2n_session_ticket_test.c                          ... NOTE: Some details are omitted, run with S2N_PRINT_STACKTRACE=1 for a verbose backtrace.
See https://github.com/awslabs/s2n/blob/master/docs/USAGE-GUIDE.md
FAILED test 310
(memcmp( (serialized_session_state + 1 + 2), (ticket_key_name2), (strlen((char *)ticket_key_name2)) )) == (0) is not true  (s2n_session_ticket_test.c line 774)
Error Message: 'no error'
 Debug String: 'Error encountered in s2n_recv.c line 69'
 System Error: Socket operation on non-socket (88)
Makefile:63: recipe for target 's2n_session_ticket_test' failed
make[2]: *** [s2n_session_ticket_test] Error 1
make[2]: *** Waiting for unfinished jobs....

Proposed Solution:

Investigate and fix the root cause of the flaky test.

[maddeleine]

I recently merged a fix for this test, should have caused this specific instance of flakiness to go away: #3913.