Tests: Update unit tests to account for lazy-loading being disabled for custom logos by default.

Follow-up to [48870].

Merges [48874] to the 5.5 branch.
See #50933.
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git-svn-id: https://core.svn.wordpress.org/branches/5.5@48637 1a063a9b-81f0-0310-95a4-ce76da25c4cd