feat: add testing framework with reference project integration

- Add test/ directory with test harness and test files
- Add UDT, DB, FB parser tests
- Add PLC JSON and integration tests
- Test against reference project (239 UDTs, 12 DBs, 37 SCL, 13 XML)
- Fix attribute parsing to handle underscores (S7_SetPoint)
- Add ORGANIZATION_BLOCK support to FB parser

test/run_tests.lua

@@ -0,0 +1,47 @@
+#!/usr/bin/env lua
+
+package.path = package.path .. ";lua/?.lua;src/?.lua"
+
+local test = dofile("test/test_harness.lua")
+
+print("========================================")
+print("   SCL LSP - Test Suite")
+print("========================================")
+print(string.format("Reference Project: %s", test.REF_PROJECT))
+
+local function run_all_tests()
+  print("\n" .. string.rep("=", 40))
+  print("Running All Tests")
+  print(string.rep("=", 40))
+
+  test.reset()
+
+  local function run_test_file(name)
+    print(string.format("\n>>> Running %s", name))
+    local ok, err = pcall(dofile, "test/" .. name)
+    if not ok then
+      print("ERROR: " .. tostring(err))
+      test.fail_count = test.fail_count + 1
+    end
+  end
+
+  run_test_file("test_udt.lua")
+  run_test_file("test_db.lua")
+  run_test_file("test_fb.lua")
+  run_test_file("test_plc_json.lua")
+  run_test_file("test_integration.lua")
+
+  test.report()
+
+  return test.fail_count == 0
+end
+
+local success = run_all_tests()
+
+if success then
+  print("\n=== ALL TESTS PASSED ===")
+  os.exit(0)
+else
+  print("\n=== SOME TESTS FAILED ===")
+  os.exit(1)
+end