Address the first item in #161456 (Unit tests in the display_list/ directory)

Some new DlPath::Make<Shape> factories were added to make test writing simpler.

DlPath is now bi-directional! You can construct one from either an SkPath or an impeller::Path and it will auto-convert to the other as needed. This allows unit tests with custom paths to rely on impeller::Path for path construction instead of SkPath (as long as only simple move/line/quad/curve verbs are needed).

RoundRect now normalizes the argument rect in all constructors to match Flutter expectations and SkRRect legacy behavior. This behavior was already being enforced in ui.rrect but the unit tests we have to verify the behavior are written against the RoundRect object itself so this was the simplest way to make the unit tests work right, while ensuring that we maintain correct behavior for ui objects. Ideally these issues would be tested at the ui native interface instead of as unit tests on our internal objects and we should be allowed to decide how we want our internal APIs to behave with regard to this concept.

Skia inverted path types are no longer allowed in DlPath and all use of them should be eliminated in the engine (except to test if they crash when used in a debug unit test)

A couple of unit tests for DlOpSpy and Impeller's interop package were migrated here along for the ride even though this PR was focused primarily on display_list/ unit tests.