Previously, I used the Android emulator for testing and everything
seemed to work fine with stdout (if I remember correctly). But our
devicelab uses real Android devices and the warnings are routed to
stderr. Hence change stdout to stderr in the test.

## Description

Print actionable warnings if `flutter drive` (where most of our performance benchmarks come from) is run in debug mode and it tries to gather benchmarks using `traceAction`.

## Related Issues

https://github.com/flutter/flutter/issues/30625

## Tests

I added the following tests:
* drive_perf_debug_warning devicelab test