Ensure Meta_Test_Suite_Tests can run on the CI

[wdanilo]

This task is automatically imported from the old Task Issue Board and it was originally created by Radosław Waśko.
Original issue is here.

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The Meta_Test_Suite_Tests currently pass the enviornment variables to the children without any modifications.

Thus, if there is some test config defined for the parent test, it will be inherited by children. This is not correct - the tester should have control of what env vars are given to the children and should ensure they are consistent regardless of the parent's environment.

To do so, we need to extend System.create_process to allow to unset some environment variables. And possibly to also override some.

This way the runner will have control of child's env and will be able to run them predictably regardless of the external environment.