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 https://github.com/flutter/flutter/issues/83562