Fix known memory leaks in self-talk tests

[lrstewart]

Problem:

We have this issue to figure out why valgrind doesn't find memory leaks, but in the meantime, I've had multiple PRs fail valgrind for random self-talk tests I didn't touch.

We should fix all the self-talk tests, which almost all have the same memory leak. They allocate memory for certs, but only free that memory in the server process, not the client process. Example: https://github.com/awslabs/s2n/blob/master/tests/unit/s2n_self_talk_test.c#L118-L119

Proposed Solution:

Go through all the tests/unit/s2n_self_talk_* tests and either:

• Move the memory they allocate for certs to the stack. Example: lrstewart@7ed37b4
• Call the common testlib function to setup the cert chain in the server process after the fork. Clean up at the end of the server process.