Tweak the primary flutter build rule so that fuchsia is more similar to
other platforms in how tests and the shell are built.
Only embedder_unittests and GLFW tests are disabled on Fuchsia now.
TEST: Ran unittests on host/fuchsia; workstation on fuchsia
BUG: fxb/53847, fxb/54056
This does not actually import the runners into the engine. It only sets up the targets so they need no modifications are necessary when the migration is done. The engine has been verified to build in both buildroots.
