libgbl: migrate fuchsia_boot tests to FakeGblOps

Switch fuchsia_boot tests to using the common FakeGblOps.

As part of this, the underlying AvbOps now uses the upstream test
fixture provided by libavb, which has some additional features and
prevents us from having to duplicate some of this logic.

Bug: b/366550869
Change-Id: Ie6b29c2e4a4361952ba0a296efb2c28a356065cb