Refactor cipher tests

[christophhagen]

The group cipher and session cipher tests can be simplified:

test_group_cipher

test_no_session()

• bob_session_builder is unused and can be removed

test_session_cipher

initialize_sessions_v3()

• alice_ephemeral_key is unused and can be removed
• bob_pre_key is unused and can be removed

  generate_test_message_collections()

• Use parameter size consistently (instead of 50)
• Simplify code by calling decrypt_and_compare_messages (twice)

  run_interaction()

• Switch alice_address and bob_address for consistency (Alice should initialize session for Bob's address)