Add Promise.withResolvers polyfill for latest PDF.js version

[toofar]

• Add polyfill of Promise.withResolvers for recent versions of PDF.js.
• Enable some existing PDF.js related tests (in qutescheme.feature) in just a couple of CI environments.
• Fix a unit test for newer PDF.js versions (oh, actually it was the real version parsing code, that the unit test was testing)
• Adapt update_3rdparty.py to only import stuff from qutebrowser if needed, due to some weirdness trying to run it in the bleeding edge test setup

I tested the docker container locally and the relevant tests passed.

Closes: #8170

[The-Compiler]

Looks great, thanks!