Test: standardize tests for all matchers

Clear and concise description of the problem

Currently, matchers tests are all over the place. We do not have tests for all possible errors produced by matchers.

Suggested solution

I think we should have a separate test/matchers folder. Tests for all matchers should also include the final message, not just call the matcher and be done with it. We want to make sure the error message is helpful.

test('matcher works', () => {
  expect(2).not.toBe(1) // bad test

  // better test -- maybe even use fuzzy
  const error = throws(() => expect(2).toBe(1))
  expect(error.diff).toBe('...')
  expect(error.message).toBe('...')
  expect(error.actual).toBe('2')
  expect(error.expected).toBe('1')
})

Having coverage for it should help us catch all the missing tests too.

Alternative

No response

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

Start by inventorying the existing matcher tests and compare their locations with the proposed test/matchers folder. Run the matcher test suite, then standardize coverage so each matcher checks its final message, diff, actual value, and expected value where applicable; done means missing error cases are covered.