Resolve "Fix Modbus Server Port in Tests"

Summary

Use config_modbus_port instead of always using 502. Also some refactoring in the tests.

Explain your context

(Explain what you did.)

Please list breaking changes, new features or bugfixes

• Breaking changes:

• New features:

• Bugfixes:

  • Modbus tests run on Linux/Mac without root access

Closes #259 (closed)