tests: added netfilter tests

[themighty1]

This PR adds integration tests for the netfilter functionality.

• Added netfilter integration tests
• Refactored iptables rule building into a dedicated function to eliminate code duplication between production and test code
• Extracted network test harness into a shared module used by both eBPF and netfilter tests

The production code was modified to extract rule-building logic into a testable function. This approach maintains a single source of truth, but I'm open to splitting the production changes into a separate PR if preferred.

[gustavo-iniguez-goya]

perfect 👌 !

By the way, when we added OpenSnitch to Debian I disabled some tests that require kernel level privileges, because they cannot run on their tests environment, which are chroots or LXC containers.
I added PRIVILEGED_TESTS= to skip these tests, disabled by default. We'll run into the same issues with these tests that require access to the kernel if there's no way to disable them.

[themighty1]

That's a good observation, thanks.
I will keep that in mind and also will check Debian testing procedures.