Integration tests don't check screenshots #2918

The integration tests don't check screenshots. They fetch the event the test app printed and check its fields, but not its screenshot.jpg, so a broken or empty screenshot goes unnoticed. This let the Metal readback error fixed in #2917 pass CI: the iOS simulator and macOS runs logged it on every capture.

The screenshots are empty in CI on every platform anyway. The test app calls Application.Quit in the frame it captures in, so the end-of-frame screenshot never gets taken and the SDK sends a 0-byte screenshot.jpg ("Screenshot was not taken in time").

What we could do:

• Let IntegrationTester wait for the screenshot before quitting in the message and exception tests.
• Assert that the event's screenshot.jpg isn't empty, skipped where no screenshot is taken (batch mode, WebGL). Reading attachments needs a function in app-runner's sentry-api-client, which has none.