Add a test to show that when PATCHing an object with current data, doesn't fire any modification events

[erral]

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See plone/plone.namedfile#216 and plone/plone.scale#148 and related PRs.

We are using the REST API to import content to a given site.

When importing the lead image of a content, we have seen that our site's Data.fs is growing and growing.

We have traced this down to plone.namedfile lacking a __eq__ method, which is to be used when comparing field value equality before setting the content

This test shows that no modification events are fired when trying to PATCH a image field with the same image.

The tests should fail, but pass when run together with plone/plone.namedfile#216 in Jenkins

[mister-roboto]

@erral thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!