Move asset_bundle_package_test to MemoryFileSystem to avoid setting fs.currentDirectory in tests (#21427)

* Update asset_bundle_package_test to use MemoryFileSystem

This updates all tests in this file to use a memory filesystem (same as I did for asset_bundle_package_font_test) based on the current platform.

There were already a few minor tests at the bottom using a MemoryFileSystem and being run for all platforms - however this complicated the shared code I changed above to support Windows natively so I changed them to also work the same as others (that is, they just run the native platforms so require running on all to get full coverage - though this is the case for almost all other tests anyway).

* Move fixPath to the correct place