See flutter/packages#3046 for context, where I have tried a number of different things to debug this.

The last of the macOS host Cirrus tests in flutter/packages is the "custom tests" bot, which among other things run's Pigeon's integration tests. Trying to move those tests to LUCI causes them to hang and time out. So far what I've found is:

• It's only the macOS tests (which involve launching a macOS app), not the iOS tests (which involve launching an iOS simulator).
• If I run either of the macOS tests by itself on LUCI, everything is fine. If I run that same test after one of the iOS tests, or after the other macOS test, the hang happens.
• There doesn't appear to be a sub-process left open.

My last working theory was that there was a bug in the Pigeon Dart test harness with awaiting the process and/or its streams, but variations on that code (not doing output streaming at all, switching from custom code to the battle-hardened process_runner package) haven't helped, so I'm currently not sure what to try next.

This blocks completing the macOS-host LUCI transition for flutter/packages.