Refactor the pps posttroll messaging hook and add unittests

[adybbroe]

Refactoring of the pps posttroll messaging hook. Adding unittests

[ghost]

DeepCode's analysis on #46b23d found:

• :information_source: 6 minor issues. :point_down:
• :heavy_check_mark: 1 issue was fixed.

  Top issues

Description	Example fixes
Unused PPSMessage imported from nwcsafpps_runner.pps_posttroll_hook Occurrences: test_pps_hook.py:66	:wrench: Example fixes
Access to a protected member _to_send of a client class Occurrences: test_pps_hook.py:352 test_pps_hook.py:356 test_pps_hook.py:358	:wrench: Example fixes
standard import "import unittest" should be placed before "import pytest" Occurrences: test_pps_hook.py:31 test_pps_hook.py:32	:wrench: Example fixes

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[adybbroe]

Okay, I merge this now. It has been tested at SMHI for a while now, and it seem to work