Require field names in struct literals

Require field names in struct literals

Struct literals use field names, with one field on each line when more
than one field is set, and table-driven test cases follow the same rule.
The old rule applied only to keyed literals, so positional test cases
with many fields on one line passed it. A small two-field type that acts
as one value, such as a node key, can still use a positional literal.