Fix the "caches image resources at the document/page level as expected (issue 11878)" unit test #18404
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This unit test fails occasionally (albeit much less than before thanks to PR #17663), so we change the parsing time check's divisor to prevent it from happening again. If the last page's rendering time is less than or equal to 50% of the first page's rendering time that should be enough proof that no worker thread re-parsing occurred while also providing a wide enough range to avoid intermittents.
Note that the assertion is now equal to the one we already have in the "caches image resources at the document/page level, with main-thread copying of complex images (issue 11518)" unit test which seems to work reliably so far.
Fixes #18403.