Arguably we maybe don't need to test them even in wasm2js, as the wasm backend
lowers them to something that the wasm2js should not have any special handling for,
but I think it's worth keeping that testing around to be safe.

Also enable test_nested_struct_varargs which has been disabled since the beginning
of time, because it failed in fastcomp, but it has passed in the wasm backend (since
forever, I suspect, as the issue was legalization, which was never a problem for the
wasm backend).