Add basic tests ensuring that examples continue to function

[shyndman]

A user recently reported that the todo example wasn't working, and it would be good to ensure that it doesn't happen again

[chonghorizons]

I can report that the todos doesn't work. I was looking for a way to see if the graphql/Hasura connection was established, but the NhostClient(baseUrl: nhostApiUrl) object doesn't have a checkStatus method that I could figure out.

I get this:

when running packages/nhost_flutter_graphql/example/lib/simple_graphql_example.dart from Android Studio.

• No linter errors.
• Yes, I made an app and changed the two URLs
• Yes, I created a todos table.
• I think I correctly change the permissions to public. I was running the example to see if it's working. Example won't run, but I'm not sure if this is because the endpoint is still not accessible OR the example code has an error.
• Note: I was building for web, not an Android device.