Fix `ConfigTests` tests

[Gor027]

Added implementation for functions in testing.cpp used in ConfigTests tests:

• get_connect_timeout_from_cluster
• get_port_from_cluster
• get_contact_points_from_cluster

To avoid generating bindings for functions that are directly under namespaces and may return objects of custom types, the testing_utils header is added as a median between the Rust implementation of those functions and the testing.hpp header.

Added ConfigTests tests to GitHub Actions for Cassandra and ScyllaDB.

Pre-review checklist

• [x] I have split my patch into logically separate commits.
• [x] All commit messages clearly explain what they change and why.
• [x] PR description sums up the changes and reasons why they should be introduced.
• [x] I have enabled appropriate tests in .github/workflows/build.yml in gtest_filter.
• [x] I have enabled appropriate tests in .github/workflows/cassandra.yml in gtest_filter.