The 'reports buffering status' test was written in an unnecessary complicated way that used pairs of completers and booleans that tracked the same thing, and more importantly had a bug (hidden by the fact that this package is still on legacy analysis options) where the `await` statements weren't actually doing anything, because they were not actually `await`ing futures.

This fixes the lack of awaits—which should fix the flake—and removes the unnecessary booleans.

Fixes flutter/flutter#94775

Currently the flake situation for Firebase Test Lab tests is very bad,
and the task running those tests are some of the slowest tasks in the
CI. Re-running failed tests is slow, manual work that is signficantly
affecting productivity.

There are plans to actually address the flake in the short-to-medium
term, but in the immediate term this will improve the CI situation, as
well as reducing the drag on engineering time that could be spent on the
root causes.

Part of flutter/flutter#95063

…4618)

…oading files (#4601)

This task's memory usage is very close to the 4GB limit of the light-workload machines, and sometimes goes over and causes flaky failures.

This is a breaking change to video_player_platform_interface to remove Pigeon mocks from the public interface. These should never have been there, as tests in packages outside of the platform interface should be mocking the platform interface rather than the method channel, but the app-facing tests weren't rewritten when they should have been so they ended up published to support the legacy tests. This creates an unwanted non-dev dependency on `flutter_test`, in addition to promoting an anti-pattern.

While making the breaking change, this also adopts `PlatformInterface`, removing `isMock` in favor of the standard mixin pattern provided by the base class.

Part of flutter/flutter#83562

…hread (#4608)

Allows the other video_player packages to use either 4.x or 5.x of
`video_player_platform_interface`, since the only breaking change
doesn't affect them (as it just removes test code that they no longer
use). This allows clients of `video_player` to no longer have a
transitive dependency on test packages, but doesn't create any version
lock with any unendorsed implementations that might exist.

Fixes flutter/flutter#83562

…thread" (#4630)