Skip unit tests with tuist test

We've had already had a way to easily skip UI tests with a single flag in tuist test:

tuist test --skip-ui-tests

Now, we're bringing the some convenience flag for unit tests:

tuist test --skip-unit-tests

This way, you can easily have separate CI pipelines for UI and unit tests, such as when you don't want to run your UI tests for every PR but instead nightly.