Refactor tests and improve packet normalizer

- Move all test files to tests/ directory
- Update Makefile to build tests in tests/ directory
- Add fragment reassembly timeout mechanism (500ms default)
- Add error count API (pkt_normalizer_get_error_count, pkt_normalizer_reset_error_count)
- Enhance error handling with timeout callbacks
- Add comprehensive stress tests and edge case tests
- Update .gitignore for build artifacts

tinycrypt/.gitignore

@@ -0,0 +1,5 @@
+*.o
+*~
+*.d
+*.exe
+*.a