What this MR does / why does DuMux need it:
Multithreaded assembly with the pq1bubble couplingmanager does not work at the moment, see #1190 (closed).
This MR turns of multithreaded assembly for this case and corrects the reference solution of
test_ff_stokes_channel_3d_nonuniform_pq1bubble_simplex, previously computed with the faulty multithreaded setup.
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