duration_based_chunks algorithm produces empty groups, causing pytest to fail when collecting tests

Something in the most recent set of changes has broken something. Previously I have been doing --splits 20 and all of the groups have at least one test. Now, at least the last two groups are empty, which causes pytest to exit with error 5 and github actions to kill all of the tests. Not sure what the precise cause of this is at the moment.

Start by reproducing the failure with --splits 20 and inspect the duration_based_chunks algorithm that creates the groups. Trace why the final groups are empty, then verify that pytest collection succeeds with no empty groups and the GitHub Actions test run is no longer terminated by error 5.

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