iox-#28 Add test-threads option to nextest config file

[elBoberido]

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With the new release of the nextest runner, it now supports setting the test threads via config file. https://github.com/nextest-rs/nextest/releases/tag/nextest-runner-0.15.0

The developer must not take care to run cargo nextest run -j1 but can just run cargo nextest run

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References

• Closes #28