Unstable PFW test related to filters loading

[f1ames]

Type of report

Failing tests

Provide detailed reproduction steps (if any)

Some PFW tests fails randomly, for example see https://travis-ci.org/ckeditor/ckeditor4/jobs/585973103 #3424 (comment). It looks like there might be some inconsistency / race condition when loading PFW filters and so sometimes the filters are not ready when test are executed.

[f1ames]

I think this is the same issue which was mentioned during #835 review (see #3257 (review)) regarding the same tests failing on Edge:

[f1ames]

Fixed in #3539.

[f1ames]

Reopening since it it still an issue as we seen during recent testing phase.

[Comandeer]

They seem to fail on the first test run and only in Firefox. After that, they run smoothly. Unfortunately there is only the first run on CI, so it fails there more regularly, e.g. https://app.travis-ci.com/github/ckeditor/ckeditor4/jobs/541909640 or https://app.travis-ci.com/github/ckeditor/ckeditor4/jobs/541657054.

[Comandeer]

It seems that Firefox does something weird during script loading and load event does not mean that script is loaded and executed. Now I'm experimenting with using afterscriptexecute event in the script loader and the initial results are promising.

[Comandeer]

It seems that the issue can be reproduced in Chrome after all. So probably the issue is deeper and connected with our script loader.

[sculpt0r]

That should be fixed as a part of a bigger solution, e.g. #4948

I was able to reproduce the issue (this time on Brave - chrome) only thanks to the browser plugin, which automatically refreshes the tab and stops if certain text is on the page...

All scripts are loaded, the only 404 is for favicon. Some screens, might eventually help: