Given that Roboelectric relies on class loading, that would mean rewriting a test suite lambda, putting it into a synthetic class behind the scenes, passing captured variables to its constructor, so basically changing the language at a large scale and being a nightmare to maintain.
This would save 2 to 4 lines of code (depending on formatting). But you would lose explicitness. It would also make you wonder what really happened if you see stack traces with the synthetic class you never created.