[8.x] Add a phpunit comparator for models

[olivernybroe]

This PR adds a phpunit comparator for models.

The comparator compares the models based on the model is method.
Phpunit comparators are used for the methods assertEquals and assertNotEquals.

I often encounter that I want to check if two models are the same, and then use the assertEquals, only to get an error about things not being identical. This is often caused by fields like the wasRecentlyCreated and so on.
To get around this I normally write assertTrue($model->is($otherModel)).

This solution might be BC and should have been submitted to L9, as it changes the current assertEquals result, however submitting it to L8 because it uses the official is method which is used for comparing models. If you consider this a BC i'll submit to L9 instead 👍

Another solution for this would be so instead of using the is method for comparison it uses the getAttributes arrays for comparison.

[driesvints]

Hi, I merged #38398 into 8.x to get the test suite passing again. I've put your PR in draft. Please rebase your PR against 8.x to make the tests of this PR pass. After you've done that and tests pass, mark this PR as ready for review. Thanks.

[taylorotwell]

Yeah - not sure I want to do this on a patch release. Can't even decide if I want to do it at all atm 😅

[olivernybroe]

Yeah, prob. shouldn't be patch 😄
Thought I would open a PR for hearing the teams thoughts about it 👍