This change adds a FlutterMetalRendererConfig that lets embedders
specify metal as rendering api.
Also adds a test that validates rendering a gradient using metal.
This is part of a larger effort to expose the difference between GrDirectContext,
which runs on the GPU thread and can directly perform operations like uploading
textures, and GrRecordingContext, which can only queue up work to be delivered
to the GrDirectContext later.
`//flutter/testing` now contains a lot of utilities used by other test targets.
This includes stuff like working with render targets that use either OpenGL or
Metal, fixtures for interacting with the Dart VM, test assertion predicates,
etc.. However, these utilities themselves are not tested as part of a standalone
test suite. Instead, only the test targets that include it exercise these
utilities. Since these are no longer trivial, a new test target has been added
that tests the testing utilities directly.
