Testing changes I make to Webkit efficiently

I am trying to modify WebKit for my own purposes. I do not plan to ship my modifications to the official codebase.

How would I check that the changes I make to WebKit did not break anything? For instance, I am making changes to WebKit/Source/WebCore/dom/Element.cpp and would like to test them incrementally.

By reading the documentation on using WebKit, it seems that I need to build the entire codebase by running build-webkit each time I want to test a new change I made. Given that it takes about 90 min on my laptop to run the build, this seems like an inefficient way of developing webkit.

Am I missing something obvious, on how to speed up testing my changes to WebKit? I've googled around but for some reason can't find any clear instructions on doing this.