This PR adds flag to turn leak tracking on/off for flutter tests: #138653

Leak tracking is off by default till prove of its stability.

To prove leak tracker stability we need a signal about leak related failures on master. Ideally, after each PR. If it not possible, just regular execution of flutter tests with flag enabled.

This would be command to run in flutter/packages/flutter:

flutter test --dart-define=flutter_test_config.leak_tracking=true

Ideally I would like to see output of three types:

1. (green) Tests passed
2. (red) Tests failed and there is text Which: contains leaks: in stdout
3. (yellow) Test failed without text Which: contains leaks: in stdout

For example, it can be just email with color in the subject, like:

leak_traker report after merge of PR 123: green

I am open for any trade offs and adjustments, to make it compliant with prior art add to make it easier to configure..