Remove massive duplication in http/tls tests code
The !1116 (merged) branch introduced TLS socket and tests. The tests reused existing HTTP tests, which caused massive code duplication. The goal of this ticket is to remove duplication between the following files:
- tls_client_unittests.cc
- tls_server_unittests.cc
- server_client_unittests.cc
See comments !1116 (comment 200522) and !1116 (comment 200526).