testing/libretro-genesis-plus-gx: upgrade to git20230503

This upgrades the libretro genesis-plus-gx core to the latest commit from the source repository. The reason why I'm upgrading is that loading Genesis games on my PinePhone Pro running this core was seg faulting on this line in the code . At first I tried a patch that commented out the contents of that function, and that did fix it. But then I tried upgrading to the latest commit and that also fixes the segfault. So I've pushed an MR with the updated commit here.