bench: add ability to have multiple sensors on one pin

We may want to have multiple sensors on one pin.

I can't think of a good example but I know it's been requested. Perhaps something like light luminescence and light brightness from the same photometer pin values.

The issue does not name any files, tests, or entry points. First trace how sensor pins and photometer values are represented, then define the supported multiple-sensor behavior and add coverage showing that multiple sensor readings can share one pin.

Written by the indexing model from the issue text.