macOS ARM64 CI runner often drops testbed log lines

Instead, it logs this:
```
=== Messages dropped during live streaming (use `log show` to see what they were)
```

For example, see the bottom of "Test App" in https://github.com/beeware/toga/actions/runs/10085243913/job/27885797983.

#2730 added a workaround for the test exit status line, but the fact that other lines are randomly going missing will make it more difficult to deal with other failures, or even know what they are.

Start with the linked GitHub Actions run and inspect the macOS ARM64 runner's live testbed logging. Compare the behavior with the workaround from #2730, then reproduce the missing-line condition across runs. Done means testbed logs stream without dropped lines and failures remain diagnosable.

Written by the indexing model from the issue text.