`caliper-core`: add tests for `Message` classes

[EshaanAgg]

This PR introduces the tests for the base Message class in caliper-core, and then introduces three similar test suites for the constructors of three types of messages. When approved, I can work on adding similar tests for all the message classes.

Checklist

• [X] A link to the issue/user story that the pull request relates to
• [ ] How to recreate the problem without the fix
• [ ] Design of the fix
• [ ] How to prove that the fix works
• [X] Automated tests that prove the fix keeps on working
• [ ] Documentation - any JSDoc, website, or Stackoverflow answers?

Issue/User story

Fixes part of #1557

Steps to Reproduce

1. 2. 3. 4.

Existing issues

• [ ] Stack Overflow issues
• [ ] GitHub Issues
• [ ] Discord history

Design of the fix

Validation of the fix

Automated Tests

What documentation has been provided for this pull request

[davidkel]

@aklenik Can you take a look

[davidkel]

On a quick glance, these seem ok to me, but really want @aklenik to take a look as he is more familiar with this part of the code I think