If they followed the license nothing. My uninformed knowledge is that the rust based rewrite is MIT, the originals are GPL, and you can't include GPL code in a MIT licensed project without making it GPL.
But you should be able to use a GPL test suite on an MIT-licensed program (or even a proprietary one, without the program needing to be under the GPL.
> and you can't include GPL code in a MIT licensed project without making it GPL.
Why is that? The tests are not linked to the distributed binaries. You can also distribute project sources with mixed licenses.