Add tests for diff-based scanning in lightz

Summary

See MVC: Enable Diff-Based Scanning in MRs for Fast... (&17758 - closed) on how this issue fits in the whole plan.

Also read this thread for context on the changes needed here.

References

Main diff-based scanning epic: Faster Advanced SAST: Diff-based scanning in MRs (&16790 - closed)

MVC implementation epic: MVC: Enable Diff-Based Scanning in MRs for Fast... (&17758 - closed)

Implementation Plan

1. Add tests in lightz to test the diff_scan_neighborhood_depth flag behavior with depth set to 0, 1, 2.
   • It should scan all the files specified in diff_scan_file_list_path and the neighbors of those files based on the configured depth
     • Here's an example code that I used to verify the behavior of these 2 flags

Note that for now, we do not plan to enable users to configure the diff_scan_neighborhood_depth which will be defaulted to 1(rationale see Implications for security approval section). However, we should still verify that this flag actually picks up the correct neighborhood depth with tests.