It's surprising how poorly supported Shadow DOM is across the major UI testing frameworks. To access a Shadow DOM element you'll typically need to write a JS shim that uses the DOM APIs to get at the element, and then do other tricks to work around things like closed vs. open shadow roots and handle how event propagation works with shadow DOM.

Shameless plug, but we're building a no-code testing framework (https://reflect.run) that does all of this for you automatically. Works great with Shadow DOM (both open and closed) as well as more esoteric things - for example did you know Salesforce's Lightning framework overrides how Shadow DOM works? :D