Let me state upfront that this strategy of keeping WebKit synchronized with parts of web-platform-tests has worked quite well for me, but I’m not at all an expert in this area so you might want to take advice from someone else.
Once I've identified what tests will be impacted by my changes to WebKit, including what additional coverage might be needed, I create a branch in my local web-platform-tests checkout to make the necessary changes to increase coverage. I try to be a little careful here so it'll result in a nice pull request against web-platform-tests later. I’ve been a web-platform-tests contributor quite a while longer than I’ve been a WebKit contributor so perhaps it’s not surprising that my approach to test development starts with web-platform-tests.
I then run import-w3c-tests web-platform-tests/[testsDir] -s [wptParentDir] --clean-dest-dir on the WebKit side to ensure it has the latest tests, including any changes I made. And then I usually run them and revise, as needed.
This has worked surprisingly well for a number of changes I made to date and hasn’t let me down. Two things to be mindful of:
~/Documents. You might not have a good time.
[wptParentDir] above needs to contain a directory named web-platform-tests, not wpt. This is annoyingly different from the default you get when cloning web-platform-tests (the repository was renamed to wpt at some point). Perhaps something to address in import-w3c-tests.