The following changes improve tests stability.
The `assertEqualFields()` method expects an object and a fields array as inputs and subsequently approaches the received parameters as such, but did not verify whether the received parameters are of the expected types.
Along the same lines, the `assertSameSets()`, `assertEqualSets()`, `assertSameSetsWithIndex()` and the `assertEqualSetsWithIndex()` methods all expect arrays for both the actual as well as the expected values and uses the array function `[k]sort()` on both, but never verified that the received inputs were actually arrays, which could lead to PHP errors on the sorting function calls.
Follow-up to [30687], [42343], [48937], [48939], [51480], [51481].
Props jrf.
See #53363.
git-svn-id: https://develop.svn.wordpress.org/trunk@51697 602fd350-edb4-49c9-b593-d223f7449a82