test: increase test coverage and extract test-util library

[LayZeeDK]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Test utilities are in tests subfolder of Lumberjack publishable library.

Issue Number: N/A

What is the new behavior?

Test utilities are in the @internal/test-util workspace library.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[NachoVazquez]

Excellent work! Refactoring the test-utils to its own library cleans the repo for good.

I'll continue the review tomorrow, I want to go deep into what's happening in the new tests and utils.