Hi,
For javascript unit testing of Mediawiki extensions, qunit can be used and it is documented at https://www.mediawiki.org/wiki/Manual:JavaScript_unit_testing#QUnit
We also documented(thanks to Krinkle) some guidelines on writing qunit test cases at https://www.mediawiki.org/wiki/Manual:JavaScript_unit_testing/QUnit_guidelin...
In the last sprint of i18n team, we have added qunit test cases for WebFonts extension and as far as I know no other extension use qunit for unit testing. If you are planning to write js test cases for your extension please make use of these documentation and help us to improve it.
Thanks Santhosh Thottingal