DrillSideways does not support intra-segment concurrency

With the introduction of intra-segment search concurrency (see #13542), `DrillSideways` is the only case that disables it in tests which can't support intra-segment slicing given its implementation requirements to go through all docs in a segment at once. This is asserted as well in its score method which checks that min and max are respectively `0` and `NO_MORE_DOCS`.

With intra-segment slicing randomly enabled in tests, we need to explicitly disable intra-segment concurrency in any test that relies on `DrillSideways`. That required adding an additional `newSearcher` method that takes in the supported concurrency: none, inter-segment or intra-segment.

This issue is to discuss a plan to support intra-segment concurrency in `DrillSideways`. Ideally, we'd be able to one day enable intra-segment slicing by default, once #13745 is addressed, but that would not make `IndexSearcher` usable in `DrillSideways` out of the box which is not a good user experience.

Start by reading the DrillSideways implementation and its score method, then review #13542 for intra-segment search concurrency and #13745 for the related limitation. Define a concrete design for supporting intra-segment slicing without requiring callers to disable concurrency, and document the expected behavior and tests needed to verify it.

Written by the indexing model from the issue text.