[url_launcher] Fix minor memory leak in Linux url-launcher tests

[robert-ancell]

Fix minor memory leak in Linux url-launcher tests

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki 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/plugins repo does use dart format.)
• I signed the [CLA].
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed 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].
• 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.

[stuartmorgan-g]

Oops! Thanks for the fix, LG other than needing a CHAGELOG update (using ## NEXT) to make publishable happy.

[robert-ancell]

@stuartmorgan I updated the description to avoid the need for a CHANGELOG entry - should I add one for a change like this?

[stuartmorgan-g]

The policy we'd settled on for now is to generally require a CHANGELOG even for non-production changes because it's convenient for us to see what changed (especially since occasionally we make changes we think aren't production-affecting but were wrong, although obviously that wouldn't apply here). We may revisit if it's annoying after living with it for a while.

I'm fine with not having one here, but you'll need to push something to get publish green; rerunning unfortunately doesn't work (I guess Cirrus is caching the description it had or something?)

[flutter-dashboard]

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[stuartmorgan-g]

LGTM