tls: refactor tls tests

[lizan]

Description:
From #4566 review:

• use v2 config
• use bssl::UniquePtr wherever possible
• read from file using TestEnvironment::readFileToStringForTest
• extract cert / private key read to a function

[lizan]

@ramaraochavali do you want to take this?

[ramaraochavali]

@lizan Yes. Please assign it to me