example_liddriven_cavity fails on minimal installation

@timok @DennisGlaeser

473/480 Test #480: example_ff_liddrivencavity_plot ............................***Failed    0.34 sec
Traceback (most recent call last):
  File "/data/build/dumux/examples/liddrivencavity/run_and_plot.py", line 29, in <module>
    y[str(re)], vx[str(re)] = np.genfromtxt('example_ff_liddrivencavity_re' + str(re) + '_vx' + '.log', skip_header= True).T
  File "/usr/local/lib/python3.8/dist-packages/numpy/lib/npyio.py", line 1749, in genfromtxt
    fid = np.lib._datasource.open(fname, 'rt', encoding=encoding)
  File "/usr/local/lib/python3.8/dist-packages/numpy/lib/_datasource.py", line 195, in open
    return ds.open(path, mode, encoding=encoding, newline=newline)
  File "/usr/local/lib/python3.8/dist-packages/numpy/lib/_datasource.py", line 535, in open
    raise IOError("%s not found." % path)
OSError: example_ff_liddrivencavity_re1_vx.log not found.