Don't call img.onload() manually during test as images might've already loaded

[mrxz]

Description:
One of the a-assets tests waits for an image to be loaded, but then proceeds to manually invoke img.onload(). With the change made in #5253 it's possible that this onload isn't defined, causing the test to fail. This seems to happen consistently with Firefox 115, but not on Chrome or older Firefox versions.

There is actually no need for the test to call onload. It might've been done in an attempt to speed the loading up(?), but that partially invalidates the test, not to mention that other tests do wait on the underlying assets to load naturally.

Fixes #5095

Changes proposed:

• Don't manually invoke img.onload(), but instead rely on the actual loading mechanism (either short-circuiting if already completed or onload)

[dmarcos]

Thanks!