Missing unit test for .psis_subset() output structure

[utkarshpawade]

.psis_subset() manually manipulates psis object internals because loo doesn't expose a public subset method (https://github.com/stan-dev/bayesplot/blob/master/R/ppc-loo.R#L722-L738). The existing tests only check that plotting works with a subset and that oversized subsets error, nothing verifies the returned object actually has the correct structure.

If loo changes its psis internals, this will break silently.

We should add a test that checks .psis_subset() preserves class, dimensions, field values, and attributes.

[jgabry]

Closing for reason mentioned in #519 (review)