[url_launcher] and [url_launcher_platform_interface] Fix tests broken my ChannelBuffers migration

[Hixie]

Combines #3965 and #3963:

• This test broke because it relies on implementation details that are no longer valid. We believe it is redundant with the test above it and the test in the interface package at this point.
• ChannelBuffers is actually async so we have to run the test using runAsync otherwise the future will never complete.

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.
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• 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.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

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[stuartmorgan]

LGTM

[stuartmorgan]

Landing on red to fix the tree.