initTestRenderer function
Installs a FlutterRenderer on container so slot widgets can
render. The renderer is per-container — call this on each container
built by a test that isn't going through ArmatureApp (which sets
its own renderer during initState).
Implementation
void initTestRenderer(
AppContainer container, {
FlutterRendererOptions? options,
}) {
container.setRenderer(
FlutterRenderer(options: options ?? FlutterRendererOptions()),
);
}