Fix testGetItem test that rarely failed.

The failure happens when F.shape[1] == 1 (F is generated randomly).

E
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ERROR: testGetItem (__main__.TestFaustPy)
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Traceback (most recent call last):
  File "/home/gitlab-runner/test_FaustPy.py", line 300, in testGetItem
    rand_i, rand_j = self.r.randint(0,self.F.shape[0]-2),self.r.randint(0,self.F.shape[1]-2)
  File "/usr/lib64/python3.9/random.py", line 338, in randint
    return self.randrange(a, b+1)
  File "/usr/lib64/python3.9/random.py", line 316, in randrange
    raise ValueError("empty range for randrange() (%d, %d, %d)" % (istart, istop, width))