test: wait for lazy loaded images before taking visual test screenshots

[gharlan]

The mediapool thumbnail in mediapool/pages/media.list.php is rendered with loading="lazy" — and lazy images do not delay the load event, which is what the recorder navigates with (waitUntil: 'load'). The image itself comes from the media manager and has to be generated on the first request, so whether it made it into the screenshot was a race.

That is what keeps rewriting mediapool_media.png, e.g. in #6655: one commit sets light + dark to "no thumbnail", the next one puts dark back, the one after that light. Light and dark are taken from a single page load one after the other, so the image simply arrived in between. Since MIN_DIFF_PIXELS = 1 commits any pixel difference as the new golden sample, the auto-commit then retriggers the workflow and it keeps swinging.

Reproduced in isolation with a deliberately slow image response:

with loading="lazy":     after load event, t=  70ms  [{"complete":false,"w":0}]
without loading="lazy":  after load event, t=1548ms  [{"complete":true,"w":1200}]

The fix

Before each screenshot, all images are switched to eager and awaited:

• loading = 'eager' is what gets a lazy image to load at all — a fullPage screenshot doesn't scroll it into view either, so images below the fold would never have loaded.
• waitForFunction waits with a real timeout, so a permanently stuck image fails the action instead of hanging it for 30 minutes.
• decode() closes the remaining gap between "loaded" and "decoded for the next paint".

loading="lazy" appears in exactly those two thumbnail lines in the whole backend, so nothing else is affected.

The first run with this may still commit a golden sample once — namely if the current one happens to be stuck in a "no thumbnail" state. That's the last correction, not the next swing.

Noticed on 6.x, where the media manager rebuild (#6566) makes generating the image slow enough for the race to tip the wrong way more often. The cause is the same here, so this is meant to be merged up.