tests: add unit test infrastructure

pytest-based test suite with fixtures for database testing.
Covers misc.py utilities, dbs.py operations, and fetch.py validation.
Includes mock_network.py for future network testing.

tests/fixtures/sample_json.txt

@@ -0,0 +1,5 @@
+[
+  {"ip": "1.2.3.4", "port": 8080, "type": "socks5"},
+  {"ip": "5.6.7.8", "port": 3128, "type": "http"},
+  {"host": "9.10.11.12", "port": 1080, "protocol": "socks4"}
+]