fixtures: avoid mutable arg2index state in favor of looking up the request chain

[bluetech]

pytest allows a fixture to request its own name (directly or indirectly), in which case the fixture with the same name but one level up is used.

To know which fixture should be used next, pytest keeps a mutable item-global dict _arg2index which maintains this state. This is not great:

• Mutable state like this is hard to understand and reason about.

• It is conceptually buggy; the indexing is global (e.g. if requesting fix1 and fix2, the indexing is shared between them), but actually different branches of the subrequest tree should not affect each other.

  This is not an issue in practice because pytest keeps a cache of the fixturedefs it resolved anyway (_fixture_defs), but if the cache is removed it becomes evident.

Instead of the _arg2index state, count how many argnames deep we are in the subrequest tree ("the fixture stack") and use that for the index.

This way, no global mutable state and the logic is very localized and easier to understand.

This is slower, however fixture stacks should not be so deep that this matters much, I hope.