[devtools] Make timeout error in qunit tests configurable and use SkipTest in test_jscript.py The reason is that the latter tests quite often fail in our CI environment, possibly due to concurrent execution of the browser, and this makes it hard to identify a truly failing build there. So skip tests with a timeout error.

cubicweb source: source-is-missing cubicweb/web/data/jquery.flot.js line length is * characters (>512)