Add tests for `flame_texturepacker`

[spydon]

What could be improved

Add tests for flame_texturepacker.

Why should this be improved

Because we should have a well tested codebase.

Any risks?

No.

More information

https://github.com/flame-engine/flame/tree/main/packages/flame_texturepacker

[spydon]

@ycfay this is not related to the issue you posted in, please open a new one (and make sure to format your code properly).