`rabbit_net:fast_close/2`: review tweaks

The closer process now catches the exit `ssl:close/2` throws
when the stuck TLS connection is killed as a last resort, so
that path no longer produce a crash report in the logs.

Tweak tests along the way to not depend on rare
single-platform Erlang/OTP TLS socket behavior.
Yes, the complexity of this seemingly straightforward
part of the sockets API goes even deeper :(