[fix/adaptedVariousStokesThresholds] Updated thresholds for tests

Merged Katharina Heck requested to merge fix/adaptedVariousStokesThresholds into master

Updated thresholds for test_stokes, test_2cstokes2p2c, test_2cnistokes2p2cni and test_2cnistokes2p2cni_boundarylayer to make them pass.

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