Add several test cases for built-in TLS server feature

[supercaracal]

Hello,

Redis 6 supports SSL/TLS connection optionally, so I would say that it would be better to test it in this repository.
Howerver, since the feature is optional, I added the cases to CI workflow as a additional job.

I hope this will help. But I will close the PR if the test cases are neither useful nor meaningful.

• https://redis.io/topics/encryption
• https://github.com/redis/redis/blob/unstable/TLS.md
• https://github.com/redis/redis-rb#ssltls-support

ref: #972