See Layout Tests for general info on how to build and run layout tests.
There are two styles of accessibility layout tests:
Use the unit-style tests. An example is aria-modal.html.
These tests check the accessibility tree directly in Blink using
AccessibilityController, which is just a test helper.
The code that implements the bindings is here:
You‘ll probably find bindings for the features you want to test already. If not, it’s not hard to add new ones.