Add helper tests for missing enum variants

[FreezyLemon]

Often, there are enums in the ffmpeg crate that have From<AVEnum> for Enum implementations (and vice versa). These will automatically show missing variants due to non-exhaustive match statements whenever a new FFmpeg version is released.

However, there are some enums where this doesn't work. Those could (and should) have a test, hidden behind a feature flag, to check whether the match statement on the AVEnum is exhaustive. Rough draft:

#[cfg(all(test, feature = "check-enums"))]
mod dev_test {
    fn exhaustive_av_option_type(x: AVOptionType) {
        match x {
            // all variants
        }
    }
}

(AVOptionType is a random example, it currently has an exhaustive match statement in its API). I think bitflags structs mostly benefit from this.

If someone starts adding these, they might aswell add these for all known enums while they're at it (and offload them to a separate test-only file). That would make it easy to just run cargo check -F check-enum or something whenever preparing for a new FFmpeg version.