Ability to configure coverage to run by default in dart_test.yaml

[Loren Segal]

Currently the only way to run tests with coverage is to add --coverage to the command line when executing. While this works, it does not play nice with other tools that may be in control of the flutter test command, like VS Code extensions.

It would be nice to be able to configure coverage values in dart_test.yaml or otherwise be able to provide --coverage and other default arguments to every run of the test command.