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.
2a44203d96