[Meta] Fix intermittent issues in the unit tests

[timvandermeij]

Despite having fixed the most occurring intermittent failures in the unit tests (see the closed issues with the intermittent label) we unfortunately still have a few of them left that trigger from time to time. We should list and fix them. This issue is a meta issue for this effort.

For reproducing unit test failures it can be very useful to use the same browser and random seed as in the logs. This ensures that the tests are run in the exact same environment and order as on the bots, which could help in case of tests that depend on the order in which they are run.

The following intermittents have been found on the bots as of July 11th, 2024, which is after landing all previously identified improvements:

Name	OS	Logs	Fixed?	Notes
creates pdf doc from non-existent URL	Windows	http://54.193.163.58:8877/1bb752f419a5b15/output.txt	-	Thanks to dumpio:true we now have an extra pointer for this long-lasting intermittent, namely the line JavaScript error: , line 0: uncaught exception: Object just above it. It looks like we're rejecting a promise somewhere but not catching the rejection properly.
caches image resources at the document/page level as expected (issue 11878)	Windows	http://54.193.163.58:8877/c5f2dbf1d73429e/output.txt	-	This was improved in #17663 and #18404, but sadly it's not enough apparently, so we could either implement what's described in #18403 (comment) or find another way to test this more deterministically (maybe we can expose something for the tests only to assert for sure that the caches were used?).
caches image resources at the document/page level, with main-thread copying of complex images (issue 11518)	Windows	http://54.193.163.58:8877/16758c66f66ce97/output.txt	-	-
handle reading ranges with missing/invalid "Content-Range" header	Linux	http://54.241.84.105:8877/67b06e431885029/output.txt	-	Disabled in Chrome in #19237.
checks that the official PDF.js-viewer API exposes the expected functionality	Windows	http://54.193.163.58:8877/dfb0552b543d2a2/output.txt	-	-