lightbox test: Add video player regression tests

[rajveermalviya]

Adds tests mentioned in https://github.com/zulip/zulip-flutter/pull/587#discussion_r1597195082

[gnprice]

Let's start using the "buddy review" system here, now that the start of the official GSoC period is imminent and it seems like everyone's actively engaged again.

@sm-sayedi, would you do the first review on this PR?

As a reminder since we're just starting to use this system: when you get a buddy review request, please prioritize it right after important bugs and your regressions, ahead of most of your own PRs. (In particular I don't think anyone is currently working on an issue that would come ahead of doing buddy reviews.)

[sm-sayedi]

Seems pretty decent to me. The tests are written cleverly. Good job!

Just one small comment below.

[gnprice]

@sm-sayedi Thanks for that review!

@sumanthvrao I think this is ready for your review as @rajveermalviya's GSoC mentor, now that it's had a first round of buddy review and there's only one small comment outstanding. Please take a look!

Then @sm-sayedi after the thread above is resolved and buddy review is complete, please remove the "buddy review" label and add "maintainer review", and request a review from @chrisbobbe.

[rajveermalviya]

@sm-sayedi Thanks for the review, I pushed a new revision which as mentioned in https://github.com/zulip/zulip-flutter/pull/694#discussion_r1613450430 uses Stopwatch to track play/pause time instead of a Timer which entirely removes the concern of manually cancelling the timers in each test.

[rajveermalviya]

Thanks for the review @sm-sayedi, new revision pushed!

[rajveermalviya]

Thanks for the review @chrisbobbe, pushed a new revision.

[chrisbobbe]

Thanks, LGTM! Labeling for Greg's review.

[rajveermalviya]

Thanks for the review @gnprice, pushed a new revision, PTAL :)

[rajveermalviya]

Thanks for the review @gnprice, pushed a new revision, PTAL.

[rajveermalviya]

Thanks for the review @gnprice, pushed a new revision!