test: add end2end test

[w180112]

• Create end to end test shell script and add 25 tests in the script
  • Phase 1 — SetSubscriberCount gRPC→etcd Tests (Steps 1-2)
  • Phase 2 — etcd Config Sync (Steps 3–4)
  • Phase 3 — DHCP Assignment & Subscriber Count (Steps 5, 6)
  • Phase 3.5 — PPPoE Enable Status (Steps 7-9)
  • Phase 4 — LAN→WAN Traffic (Steps 10-12)
  • Phase 5 — WAN→LAN DNAT (Step 13)
  • Phase 6 — DNS Static + Reverse Ping (Step 14)
  • Phase 7 — User 1 gRPC→etcd Config Tests (Steps 15-22)
• Add Etcd config restore script for unify test configuration