benchmark

report.py: adjust expected u-test values for tests

The test used to work with scipy 1.6, but does not work with 1.7.1
I believe this is because of https://github.com/scipy/scipy/pull/4933
Since there doesn't appear that there is anything wrong with how
we call the `mannwhitneyu()`, i guess we should just expect the new values.

Notably, the tests now fail with earlier scipy versions.