bigarray: Fix flaky test in test_bigarray_indexing_1d

We compare A_[10*PS-1] (which is A_[1]) to 0, but

    A_= ndarray ((10*PS,), uint8)

and that means the array memory is not initialized. So the comparison
works sometimes and sometimes it does not.

Initialize compared element explicitly.

NOTE: A (without _) element does not need to be initialized -
because not-initialized BigArray parts read as zeros.

bigarray/tests/test_basic.py

@@ -116,6 +116,7 @@ def test_bigarray_indexing_1d():
     # index out of range
     # - element access  -> raises IndexError
     # - slice access    -> empty
+    A_[-1] = 0
     assert AA[10*PS-1] == (0,0)
     raises(IndexError, 'A_[10*PS]')
     raises(IndexError, 'A [10*PS]')