Fix image resizing assertion logic for RTE

What does this MR do and why?

Fixes image resizing on upload assertion in RTE by uploading a large enough image to cause a resize in the new UI.

Why the spec started failing in new UI?

The image being uploaded in the rspec was spec/fixtures/dk.png, this had the resolution of 460x322, while it worked in old UI, the new UI provides a broader viewport width due to fluid UI, this cause image to be uploaded in its actual size, causing the spec to fail.

How does this MR fix the failure?

We now upload spec/fixtures/retina_image.png, which has the resolution of 1326x650, this is large enough to cause the image resize on upload, setting the resolution to 663x325 and we assert this behaviour in the test.

References

https://gitlab.com/gitlab-org/quality/test-failure-issues/-/work_items/20741

Screenshots or screen recordings

NA

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