Allow one test per node

The new --maxschedchunk option is great for running test suites with few slow tests.

However, it has a limitation in that it sends two tests to each worker at the start even if --maxschedchunk=1. If tests are sorted by how long they take, the two slowest tests will end up on the same node.

https://github.com/pytest-dev/pytest-xdist/issues/876 sounds like this is not easy to change.

Use-Case

I have a test suite with roughly this kind of runtime distribution:

15 min: 2 tests
5 min: 5 tests
0.5 min: 30 tests

The test fixtures are extremely cheap, much cheaper than the tests themselves. We know pretty well how long each test takes, and sort tests accordingly.

As a workaround, we can take into account xdist's scheduling details in the sorting of tests, but this is confusing and it would be nicer to fix it here.

Start by tracing the implementation and tests for the --maxschedchunk scheduler option, then review the scheduling concern in issue #876. Reproduce the case with sorted tests and --maxschedchunk=1; done means the initial scheduling places at most one test on each worker and the behavior is covered by a regression test.