Tests: Update one more test to account for lazy-loading being disabled for custom logos by default.

Follow-up to [48870], [48874].

Props desrosj.
Merges [48878] to the 5.5 branch.
See #50933.
Built from https://develop.svn.wordpress.org/branches/5.5@48879


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48641 1a063a9b-81f0-0310-95a4-ce76da25c4cd