Add additional testing coverage for Olivero

This comes from @larowlan review #103 at #3111409: Add new Olivero frontend theme to Drupal 9.1 core as beta

Here's things in the .theme file I think we should have tests for, as they are likely to break if there are array structure changes.

1. olivero_preprocess_block (Completed in open MR)
2. olivero_theme_suggestions_form_alter (mglaman)
3. olivero_form_alter (mglaman)
4. olivero_preprocess_input (mradcliffe)
5. olivero_preprocess_field_multiple_value_form (brianperry)
6. olivero_preprocess_field__comment

These would be stable blockers in my book