ssl: refactor test helper start_server

Break it into multiple pieces and simplify:

  - Let callers pass a complete SSLContext object instead of a
    callback proc ctx_proc to mutate it.

  - Add a variant start_server_proc for tests that need finer control,
    and remove two callbacks server_proc and accept_proc.

  - Remove rescue for IOError, Errno::EBADF, Errno::EINVAL, and
    Errno::ENOTSOCK which as far as I can tell should not be possible.

Author: rhenium