red_knot_python_semantic: add invalid assignment diagnostic snapshot
This tests the diagnostic rendering of a case that wasn't previously
covered by snapshots: when unpacking fails because there are too few
values, but where the left hand side can tolerate "N or more." In the
code, this is a distinct diagnostic, so we capture it here.
(Sorry about the diff here, but it made sense to rename the other
sections and that changes the name of the snapshot file.)