addresses one of the stochastic failures noted in #864

This change reduces the frequency of the first stochastic failure noted in #864, so partially addresses that issue. We're seeing more variability in the number of OTUs from usearch61, so needed to expand the allow range in this test a little bit. I'm not exactly sure why we're seeing more variability, but I did notice that number of OTUs can differ from uclust in my notes on comparing the methods in #848. 

When I ran this test 25 times locally, I had 2 failures prior to this change and 0 failures following this change.