[video_player]: initial test coverage file() constructor api

[russellwheatley]

PR includes a test case for VideoPlayerController.file() constructor which doesn't have an integration test currently.

There are no issues addressed by this PR.

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[mikehardy]

Looks reasonable to me with just a question on web platform exclusion (or perhaps the opposite: android ios specific inclusion (in case macos is also not supported but toggled on?). Also a note on simulator not working but maybe that's not for file constructor, unknown. The homepage is dire with all the warnings!

[stuartmorgan-g]

Thanks for the submission! We're always happy to see PRs that increase test coverage. We apologize for the long delay in triaging this PR. We’re in the process of overhauling our PR triage system to respond much more quickly, as well as working through the backlog.

If you could resolve the conflicts here, we can get this landed. The version update can actually just be dropped since this is a test-only change, and thus doesn't need to be published.

[stuartmorgan-g]

I resolved the conflict; LGTM

[fluttergithubbot]

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