Add a test executor not based on ForkJoinPool

[shans96]

Overview

Adding a new executor based on #3108. This new executor will be opt-in via configuration, use a ForkJoinPool to schedule tests, and won't use instances of ForkJoinPool to execute the tests (most likely, a separate ThreadPool will be used). By making use of this executor, developers that are testing code which makes use of a ForkJoinPool won't experience issues with multiple tests being started without prior tests being finished.

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes

[marcphilipp]

@shans96 Thanks for your PR and sorry for not getting back to you at the time! Since #3685 has advanced more, I'm closing this PR.