Change serial runs in parallel regression to parallel

[kursatyurt]

Proposed Changes

Some tests in the parallel_regression.py was running in serial with the SU2_CFD command this PR changes them to default (mpirun -np 2 SU2_CFD).

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[kursatyurt]

For the sliding interface case slinc_steady, I suppose the difference is caused by interpolation, but have no idea why cht_compressible fails.

[pcarruscag]

LGTM, please go ahead and update the regression values for those tests.