[rfw] Activate leak testing

[ValentinVignal]

The package manipulates disposable objects. This PR activates leak testing to make sure disposable objects are correctly disposed.

It also fixes the memory leak warnings from the tests

See the documentation: https://github.com/dart-lang/leak_tracker/blob/main/doc%2Fleak_tracking%2FDETECT.md

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[ValentinVignal]

Since I modified only the tests, I think this PR is eligible for the tags override: no versioning needed and override: no changelog needed

[ValentinVignal]

cc @polina-c