test(nat): Remove unnecessary "tests" submodules

[qmonnet]

The whole files are already test submodules, no need to embed the tests into some yet more deeply-nested "tests" submodules.

Note: There's no other change in the commit apart from automatic re-formatting (and a comment clean-up); this commit is best reviewed with the option for ignoring whitespace changes, "-w" in git.

Moved out of #1419 because it doesn't really belong in there, and it's a pain to rebase every time we edit NAT tests.

[Copilot]

Pull request overview

This PR simplifies NAT unit test module structure by removing redundant nested tests submodules and moving test helpers/tests to the module root (guarded by cfg(test)), improving readability and making cross-module test helper reuse more straightforward.

Changes:

• Flatten nat/src/stateless/test.rs by removing the inner tests module and using #![cfg(test)] at the module level.
• Flatten nat/src/stateful/test.rs similarly and update imports to the new stateless test helper path.
• Minor comment cleanup / rustfmt-driven formatting changes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
nat/src/stateless/test.rs	Removes nested tests module; exposes helpers/tests at module root under cfg(test) and keeps shared helper build_gwconfig_from_overlay accessible to other test modules.
nat/src/stateful/test.rs	Removes nested tests module; updates references to stateless test helper and reflows test helper functions/tests at module root under cfg(test).