Algorithm for choosing ticket key needs better unit tests

[maddeleine]

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Problem:

While working on this PR #3913 I noticed that we don't have any unit tests for s2n_compute_weight_of_encrypt_decrypt_keys(). This is a pretty complicated function and we should add tests explicitly for the random float generation and the calculating of key weights. Currently the function is implicitly being tested in s2n_session_ticket_test.c through self-talk tests, which is not fine-grained enough to fully test the key selection algorithm.

Solution:

Some units tests for our ticket key selection algorithm. Additionally it may a good idea to refactor the function since it is doing several different things.