Slow performance of test serial_vs_paralel_padmm on Dell laptops

[tkoskela]

serial_vs_parallel_padmm test runs very slowly on our Dell laptops with 4 cores (+ hyper-threading). On GitHub actions runners it runs very quickly.

• Look into how the number of MPI ranks is set (number of OMP threads is hard coded in tests Number of threads hard-coded in tests #287)
• Test with same number of ranks as GH actions runners