Apply changes requested from peer review - updating jest tests

[GuySartorelli]

In peer reviewing silverstripe/silverstripe-admin#1419, there were some changes that were requested but not immediately made, because the mental effort to make the changes at the time was deemed higher than the value of the changes.

In many cases, these sorts of changes are being made going forward - for consistency, correctness, and robustness, it makes sense to tidy up and do them where they were requested as well.

The following changes are important enough that I think they warrant revisiting:

• Check for exactly one item, as per original test
  • MNT Use React Testing Library silverstripe-elemental-bannerblock#107 (comment)
  • MNT Use React Testing Library silverstripe-elemental#1052 (comment)
• Some test titles don't actually describe what's being tested - or the test doesn't do what the title says it should
  • MNT Use React Testing Library silverstripe-elemental#1052 (comment)
  • MNT Use React Testing Library silverstripe-elemental-bannerblock#107 (comment)
  • MNT Use React Testing Library silverstripe-elemental#1052 (comment)
  • MNT Use React Testing Library silverstripe-elemental#1052 (comment)
  • MNT Use React Testing Library silverstripe-elemental#1052 (comment)
  • MNT Use React Testing Library silverstripe-elemental#1052 (comment)
  • MNT Use React Testing Library silverstripe-elemental#1052 (comment)
• New test behaviour differs from the original test, or is in some way not doing the expected thing
  • MNT Use React Testing Library silverstripe-elemental#1052 (comment)
  • MNT Use React Testing Library silverstripe-elemental#1052 (comment)
  • MNT Use React Testing Library silverstripe-elemental#1052 (comment)
  • MNT Use React Testing Library silverstripe-elemental#1052 (comment)
  • MNT Use React Testing Library silverstripe-elemental#1052 (comment)

PRs

• MNT Apply changes requested from peer review silverstripe-elemental-bannerblock#111
• MNT Apply changes requested from peer review silverstripe-elemental#1058