internal/credentials/xds: add unit test for HandshakeInfo.Equal

[easwars]

We made the Equal method actual perform a deep equal instead of pointer equality. We should add tests for it.
https://github.com/grpc/grpc-go/blob/master/internal/credentials/xds/handshake_info.go#L47

[Aditya-Sood]

hi @ginayeh, can I pick this up?

[dfawley]

Let us know if you still want this @Aditya-Sood - I already assigned a few other things to you and don't want to overwhelm.

[Aditya-Sood]

hi @dfawley, yes I've responded to the action items for me on the open PRs
I will start working on #6763, can pick this up after that?

[miledxz]

@arvindbr8 I would love to take this one,
if you think it's fine feel free to assign me

[arvindbr8]

@zedGGs -- Thanks for your interest. But it seems like we have a PR in flight. I'm more than happy to reserve this to you until we have merged that one in.

[miledxz]

@arvindbr8 feel free to assign me on this since we finalised ongoing pr already