tests: parallel use only up to six cores

[michaellilltokiwa]

For my system nproc --all returns 12, hyperthreading cores are counted as well. But this makes my system a bit laggy.

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[fridis]

Does this affect the github builds? Would like to see them run as fast as possible.

[michaellilltokiwa]

@fridis Yes. I thought github runners have only one core available but this does not seem to be true (anymore). They have 2-4 cores available. I updated the logic to run up to six tests in parallel.