Move connection helpers into test_helpers

[rukai]

A step towards eliminating the #[path = "../../tests/helpers/mod.rs"] hack.

The CassandraConnection type and redis connection helper functions have been moved into a new module named: test_helpers::connection

The redis TLS helper relied on some shotover internals.
I've reimplemented that logic directly into the test to remove that dependency.

Some extra benefits:

• Allows removal of all the #[allow(dead_code)] annotations because the code is now in a regular old library crate instead of hacked in with #[path = ".."]
• I suspect it will allow us to work around the "dev-dependencies can not be optional" problem that is preventing us from making cassandra-cpp an optional dependency (but needs some extra work that I've left out of this PR)