`TestNuma` fails on `C4` machines for AMD64 Ubuntu

`TestNuma` fails with `got 4 numa nodes, want 2` on C4 machines only (the specific machine-type being `c4-highmem-192` from `shapevalidation/setup.go`). I think it may be as simple a case of bumping `numa: 2,` to `numa: 4` in `x86shapes` for `C4` but I wanted to raise it as an issue so we can confirm it is indeed a typo/oversight in `x86shapes` and not an underlying issue with Ubuntu.