I'd like to be able to specify configurations for tests. My motivating example are things like https://github.com/dart-lang/webdev/blob/5d964f2e0c3b891b99eb496f933629133e8ed2d9/dwds/test/fixtures/context.dart#L95

Many of the tests need to run Chrome with a simple application and a debug port open and then test debug commands against it. Normally these are run headless, but for debugging it's useful to have Chrome visible. Right now this is controlled using an environment variable, but that's awkward to change. If we had a debug configuration that could pass an argument, or set an environment variable, or otherwise indicate something that was visible to the running test, that would be better.