Consider migrating integration tests from emulator on GHA to another tool (eg Firebase Test Lab)

This has a few benefits:
- we still have performance problems and unexpected emulator failures when running in GHA
- we are using (valuable/expensive) macos runners in order to get the emulator to function
- Nice videos of the UI tests failing
- Access to other tests (eg, firebase has robo tests that walk the app generally looking for failures and show screenshots)

There are downsides:
- We can no-longer test against http://10.0.2.2:8080/ as a hardcoded endpoint - we would need to move this selection of endpoint into a option somewhere and default to 10.0.2.2 on developer machines, but use a deployed synapse server for the CI usecase - though a single static one would be fine, I think.
- We would need to pay for these tests if we went over a small daily limit (i've been working on a Spark plan with 10 tests per day for free) - this might make them hard to support for all changes to all community runs, for instance.
- It might be a little slower to run as we need to upload the test suite each time we build (but that's only 30s or so)