Fix some lints

[etiennebacher]

Prework

• I understand and agree to the code of conduct and the contributing guidelines.
• I have already submitted a discussion topic or issue to discuss my idea with the maintainer.

Related GitHub issues and pull requests

• Ref: [general] Some lints to fix in the package #1321

Summary

This fixes some "lints", for example using more specific expect_ functions.

There are many changes but most of them are trivial:

• use inherits() instead of "foo" %in% class(x)
• use expect_named(), expect_null(), and other variants in tests
  etc.