Refactor test template name handling

[Michael1993]

Closes #601

Proposed commit message:

Refactor test template name handling (#601 / #603)

Due to Pioneer having multiple TestTemplate based extensions
the need for a unified TestNameFormatter arose. This PR moves
CartesianTestNameFormatter to the internal package, so it may
be used by other extensions as well.

Closes: #601
PR: #603

---

PR checklist

The following checklist shall help the PR's author, the reviewers and maintainers to ensure the quality of this project.
It is based on our contributors guidelines, especially the "writing code" section.
It shall help to check for completion of the listed points.
If a point does not apply to the given PR's changes, the corresponding entry can be simply marked as done.

Documentation (general)

• There is documentation (Javadoc and site documentation; added or updated)
• There is implementation information to describe why a non-obvious source code / solution got implemented
• Only one sentence per line (especially in .adoc files)
• Javadoc uses formal style, while sites documentation may use informal style

Code

• Code adheres to code style, naming conventions etc.
• Successful tests cover all changes
• There are checks which validate correct / false usage / configuration of a functionality and there are tests to verify those checks
• Tests use AssertJ or our own PioneerAssert (which are based on AssertJ)

Contributing

• A prepared commit message exists

---

I hereby agree to the terms of the JUnit Pioneer Contributor License Agreement.

[aepfli]

lgtm