Finish user properties implementation

[andsel]

Release notes

Covered publish with user properties with tests and fixed publish of will messages.

What does this PR do?

Updated the creation of Will publish to include the user properties.
Also covered with tests.
Moved some test utility methods into abstract base class.

Why is it important/What is the impact to the user?

Enables the handling of user properties in PUBLISH and CONNECT Will message. All other cases of User Properties are ignored and remain unspecified.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works
• I have updated the Changelog if it's a feature or a fix that has to be reported

Related issues

• Closes User properties implementation #876