test/test_weighted_shuffle: enlarge epsilon

test/test_weighted_shuffle: enlarge epsilon

be more permissive to address the following failure:

test_weighted_shuffle.cc:36: Failure
The difference between float(w) / total_weight and float(f.front()) /
samples is 0.010334432125091553, which exceeds epsilon, where
float(w) / total_weight evaluates to 0.53763443231582642,
float(f.front()) / samples evaluates to 0.52730000019073486, and
epsilon evaluates to 0.0099999997764825821

Signed-off-by: Kefu Chai <kchai@redhat.com>