Dispose TestThread-Container correctly

[obligaron]

Fixes #123

Remember created TestThread-container and dispose them after test run. Added a test that ensures that TestThread is initialized and disposed only once.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue).
• [ ] New feature (non-breaking change which adds functionality).
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected).
• [ ] Performance improvement
• [ ] Refactoring (so no functional change)
• [ ] Other (docs, build config, etc)

Checklist:

• [x] I've added tests for my code. (most of the time mandatory)
• [x] I have added an entry to the changelog. (mandatory)
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.

[gasparnagy]

@obligaron thx for taking care of it. The concept was good and I kept it, but I have made some simplifications:

• the test thread container can be accessed from the test runner, so we do not need to keep a separate list of the active ones
• replaced the system tests with unit tests