Redesign samplerless reads test for read-write images to not require support for the corresponding read-only format

The tests currently always require support for the equivalent read-only format which means formats that are supported only for read-write (but not read-only) images can't be tested.

See #1016.

Start by locating the samplerless reads tests for read-write images and review issue #1016 for the related context. The work is done when formats supported for read-write but not read-only images can be tested without requiring the equivalent read-only format.

Written by the indexing model from the issue text.