Build: Make dev mode work in Karma again, serve source files from disk

Build: Make dev mode work in Karma again, serve source files from disk

PR gh-4550 added support for running ES modules & AMD tests via Karma. This
required reading the `esmodules` & `amd` props from both `QUnit.config` &
`QUnit.urlParams`. By picking these two properties manually, the `dev` one
stopped being respected while ones handled directly by QUnit were fine (like
`hidepassed`). Instead of maintaining the full list of options, the code now
iterates over QUnit URL config and handles the fallbacks in a more generic way.

Apart from that, all jQuery source & test files are now read directly from disk
instead of being cached by Karma so that one can run `grunt karma:chrome-debug`
& work on a fix without restarting that Karma run after each change. A similar
effect could have been achieved by setting `autoWatch` to `true` but then the
main Karma page runs tests in an iframe by default when
`grunt karma:chrome-debug` is run instead of relying on the current debug flow.

Closes gh-4574
Ref gh-4550