Run tests headless when not debugging and headed when debugging

[Artur-]

This is how Flow does it https://github.com/vaadin/flow/blob/master/flow-test-util/src/main/java/com/vaadin/flow/testutil/ChromeBrowserTest.java#L68 Seems like a reasonable default to avoid having to manunally set/reset headless mode

Note that this mostly/only makes sense when running locally and not on a hub. When running on e.g. Saucelabs, headless probably does not offer any additional benefits