Resolve "enable check_indices by default (at least for tests)"

What does this MR do?

This makes checking indices the default.

We can postpone releasing, as I'll also work on #62 (closed) in the near future.

Changes to public APIs

Author's checklist (required)

• If the nitrile.yml format has changed, a migration has been defined in Nitrile.Package.Migrate
• A changelog entry has been added (guidelines)
• Newly added code is documented (guidelines)
• Newly added code has a style consistent with the rest of the repository
• Intermediate commits compile (use git rebase -i main if not)

Related issues

Closes #61 (closed)