Fix error in conformance test script

[tjazerzen]

Problem

Some random process could be running on 8003.

Before:

• We would not run the mock_devrev_server.py on 8003 if a process was already running on 8003.
• But if what as running on 8003 wasn't mock server, this would be problematic as reported by @gasperzgonec

After:

• If traffic is running on 8003, stop it and start mock_devrev_server.py

The same for port 8004 and rate_limiting_proxy.py.

Linked issues:

• ISS-214893