Added missingCameraPlugin.setFlashMode test to verify if the method is called on theCamera with the given id.

Part offlutter/flutter#45297.

Pre-launch Checklist

• I read theContributor Guide and followed the process outlined there for submitting PRs.
• I read theTree Hygiene wiki page, which explains my responsibilities.
• I read and followed therelevant style guides and ranthe auto-formatter. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does usedart format.)
• I signed theCLA.
• 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.
• Iupdated pubspec.yaml with an appropriate new version according to thepub 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.