layers: Fix miscounting of descriptors for multiple stages

layers: Fix miscounting of descriptors for multiple stages

Descriptors are not intended to count multiple times against the
maxDescriptorSet* limits if they are accessible from multiple stages.

Required adjusting various subtests in CreatePipelineLayout*.

Also fixed some related issues in those tests:
- Input attachments are ONLY accessible to the fragment stage.
- Various assumptions about divisibility of the maxDescriptorSet* limits

There are still many robustness issues in these tests -- it appears we
don't hit them with real implementations, but still wrong.