fix: made tests os dependent

[hanneshapke]

Summary

• TestAppDataDir_Linux was running on all platforms and failing on darwin because it asserts Linux-specific paths (/xdg/home/..., /home/user/.kiji-proxy, /var/lib/kiji-proxy) against AppDataDir(), which short-circuits to ~/Library/Application Support/Kiji Privacy Proxy on darwin.
• Renamed paths_test.go → paths_linux_test.go so Go's filename-based build constraint scopes the test to Linux only — no //go:build tag needed.
• Added paths_darwin_test.go so the darwin branch of AppDataDir() (previously uncovered) is exercised on macOS hosts.

No behavior change in paths.go; this is test hygiene only.

Test plan

• go test ./src/backend/paths/... passes on darwin (runs TestAppDataDir_Darwin, skips Linux test via filename).
• GOOS=linux go vet ./src/backend/paths/... clean — Linux build still includes the original test.
• make test-go passes end-to-end on darwin.
• CI to confirm TestAppDataDir_Linux still runs and passes on the Linux runner.