Fix TestNecAgentMain not to call sys.exit()

test_main() was calling sys.exit() killing test running
and not allowing tests to complete in a proper way.

Drop explicit sys.exit() call from net agent's main() and
fix outdated mocks in test_main().

Fixes bug #1192270

Change-Id: Iea8d5508f73eb9144e452de3305f2dec4c2a319c
Merge "Fix TestNecAgentMain not to call sys.exit()"