Add Nested annotation in Demo where it's missing

[nipafx]

Copy of #777 (so credit goes to @IlyasYOY) in a branch we can push to.

Proposed commit message:

Run more demos as part of the build (#791)

Some demo methods and classes weren't correctly annotated with `@Test`
or `@Nested` respectively and were thus not executed as part of the
build even though they should've been. For tests/classes that were not
supposed to be executed, that was not always obvious.

This change fixes that.

PR: #791

---

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
• [-] Site documentation has its own .adoc file in the docs folder, e.g. docs/report-entries.adoc
• [-] Site documentation in .adoc file references demo in src/demo/java instead of containing code blocks as text
• [-] Only one sentence per line (especially in .adoc files)
• [-] Javadoc uses formal style, while sites documentation may use informal style

Documentation (new extension)

• [-] The docs/docs-nav.yml navigation has an entry for the new extension
• [-] The package-info.java contains information about the new extension

Code (general)

• 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)

Code (new package)

• [-] The new package is exported in module-info.java
• [-] The new package is also present in the tests
• [-] The new package is opened for reflection to JUnit 5 in module-info.java
• [-] The new package is listed in the contribution guide

Contributing

• A prepared commit message exists
• [-] The list of contributions inside README.md mentions the new contribution (real name optional)