test_avg_std_and_npoints fails flakily

See [here](https://dev.azure.com/python-adaptive/adaptive/_build/results?buildId=1306&view=logs&j=88437ce6-96ce-598b-fefb-346ab1c401a0&t=cd4a0a69-2aff-5683-8f13-fd1b0f42a2d9).

The test was marked as "flaky". IMO this is a stop-gap solution and we should instead put a better error bound, for example [as shown here](http://www.ti3.tu-harburg.de/paper/rump/Ru11.pdf)