If this is an isolated incident, I vote for fixing the test. The print output is still deterministic, it's just that now it's based on source order rather than sort order.
We could just sort when printing, but then the output doesn't match the API order.
We could keep the (original) sorted order, but that would require a bit more work when sorting (we need to sort positions as well). It doesn't seem worth the effort. The go/types API doesn't make any guarantees with respect to the ordering of embedded interfaces, I believe.