Upgrade upgrade test to use pulumitest (#490)

We're deprecating providertest.NewProviderTest, so use the new
pulumitest based approach instead.

- Update test code to use PreviewProviderUpgrade.
- Use provider attachment instead of the bin directory.
- Rename state.json to stack.json & let the first run migrate the stack
to a stable, non-machine dependent name.

examples/upgrade/Pulumi.yaml

@@ -1,9 +1,5 @@
 name: provider-docker-build
 runtime: yaml
-plugins:
-  providers:
-    - name: docker-build
-      path: ../../bin
 
 resources:
   # docker buildx build -f app/Dockerfile.multiPlatform --platform plan9/amd64,plan9/386 app