Add unit tests for core functions (fixes #10)

[Larslllllll]

Summary

Adds a comprehensive unit test suite for the AdPenTest core module, covering SMTP enumeration, authentication, tool discovery, command building, and execution functions.

Tests Added

SMTP Functions

• smtp_vrfy_enum — valid users, all invalid, connection error
• smtp_rcpt_enum — valid recipients, all invalid, connection error
• smtp_auth_test — success, failure, connection error, no-TLS mode
• smtp_connect_test — return type and signature verification

AD Tool Functions

• discover_tools — all available, none available, partial availability
• build_ad_command — nmap command generation, tool-not-found error
• execute_ad_tool — successful execution, timeout handling

Test Infrastructure

• tests/conftest.py — shared pytest fixtures (mock_smtp, mock_socket)
• tests/test_core.py — 17 test cases across 6 test classes
• tests/__init__.py — package marker

Framework

• pytest with unittest.mock for mocking SMTP/LDAP/DNS services
• All 17 tests pass

Closes #10