Ensure that integration tests are actually being run

[stuartmorgan]

Many of our integration tests weren't actually being run on CI because they were in the wrong place and/or missing the driver file, and it seems we've just never noticed. This makes a number of changes:

• Ensures that all packages with integration tests also have a driver file in the right location.
  • Also standardizes the format of those files, as the boilerplate main() is available in integration_test.
• Ensures that all integration_test directories are in the right place.
  • In a couple of places, removes a duplicate copy of the integration test file.
• Makes it an error for a plugin that's not excluded to not have integration tests, so this can't easily happen again.
• Adds logging of what's being run and skipped, so if something does go wrong in the future it's easy to determine what from the logs.
  • Excludes *_platform_interface since the logging was (potentially confusingly) reporting that they were skipped because they don't support the current platform. Skipping them is correct, just not for that reason.
• Excludes the plugins that currently have no integration tests, with references to issues about adding them.

Fixes flutter/flutter#81929

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 Flutter Style Guide and the C++, Objective-C, Java style guides. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format. See plugin_tool 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.
• 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.