dolphinmainwindowtest: assert the thumbnail, not a fresh preview, after rename

testThumbnailAfterRename waited for KFileItemModelRolesUpdater::previewJobFinished
after renaming a.jpg to b.jpg. That signal is only emitted when a new preview is
generated successfully (slotGotPreview), but a rename keeps the file's content and
its already-correct thumbnail, so whether a fresh preview job runs at all is timing-
and platform-dependent. On FreeBSD CI none is produced, so the wait timed out.

Assert the actual outcome instead: the renamed item still shows a non-null preview
pixmap. This passes whether the thumbnail is regenerated, inherited across the
rename, or cache-served, and still fails if a rename ever drops the preview.

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